README: Add instructions for chain-loading U-Boot

Most Chromebooks support chain-loading U-Boot but instructions are
somewhat scattered. Add a README to hold this information within the
U-Boot tree. Also add the standard developer keys to simplify the
instructions, since they are small.

For now this only supports nyan-big.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/README.chromium

+Running U-Boot from coreboot on Chromebooks
+===========================================
+
+U-Boot can be used as a secondary boot loader in a few situations such as from
+UEFI and coreboot (see README.x86). Recent Chromebooks use coreboot even on
+ARM platforms to start up the machine.
+
+This document aims to provide a guide to booting U-Boot on a Chromebook. It
+is only a starting point, and there are many guides on the interwebs. But
+placing this information in the U-Boot tree should make it easier to find for
+those who use U-Boot habitually.
+
+Most of these platforms are supported by U-Boot natively, but it is risky to
+replace the ROM unless you have a servo board and cable to restore it with.
+
+
+For all of these the standard U-Boot build instructions apply. For example on
+ARM:
+
+   sudo apt install gcc-arm-linux-gnueabi
+   mkdir b
+   make O=b/nyan_big CROSS_COMPILE=arm-linux-gnueabi- nyan-big_defconfig all
+
+You can obtain the vbutil_kernel utility here:
+
+   https://drive.google.com/open?id=0B7WYZbZ9zd-3dHlVVXo4VXE2T0U
+
+
+Snow (Samsung ARM Chromebook)
+-----------------------------
+
+See here:
+
+https://www.chromium.org/chromium-os/firmware-porting-guide/using-nv-u-boot-on-the-samsung-arm-chromebook
+
+
+Nyan-big
+--------
+
+Compiled based on information here:
+https://lists.denx.de/pipermail/u-boot/2015-March/209530.html
+https://git.collabora.com/cgit/user/tomeu/u-boot.git/commit/?h=nyan-big
+https://lists.denx.de/pipermail/u-boot/2017-May/289491.html
+https://github.com/chromeos-nvidia-androidtv/gnu-linux-on-acer-chromebook-13#copy-data-to-the-sd-card
+
+1. Patch U-Boot
+
+Open include/configs/tegra124-common.h
+
+Change:
+
+#define CONFIG_SYS_TEXT_BASE	0x80110000
+
+to:
+
+#define CONFIG_SYS_TEXT_BASE	0x81000100
+
+
+2. Build U-Boot
+
+   mkdir b
+   make -j8 O=b/nyan-big CROSS_COMPILE=arm-linux-gnueabi- nyan-big_defconfig all
+
+
+3. Select a .its file
+
+Select something from doc/chromium which matches your board, or create your
+own.
+
+Note that the device tree node is required, even though it is not actually
+used by U-Boot. This is because the Chromebook expects to pass it to the
+kernel, and crashes if it is not present.
+
+
+4. Build and sign an image
+
+   ./b/nyan-big/tools/mkimage -f doc/chromium/nyan-big.its u-boot-chromium.fit
+   echo test >dummy.txt
+   vbutil_kernel --arch arm --keyblock doc/chromium/devkeys/kernel.keyblock \
+	--signprivate doc/chromium/devkeys/kernel_data_key.vbprivk \
+	--version 1 --config dummy.txt --vmlinuz u-boot-chromium.fit \
+	--bootloader dummy.txt --pack u-boot.kpart
+
+
+5. Prepare an SD card
+
+   DISK=/dev/sdc   # Replace with your actual SD card device
+   sudo cgpt create $DISK
+   sudo cgpt add -b 34 -s 32768 -P 1 -S 1 -t kernel $DISK
+   sudo cgpt add -b 32802 -s 2000000 -t rootfs $DISK
+   sudo gdisk $DISK   # Enter command 'w' to write a protective MBR to the disk
+
+
+6. Write U-Boot to the SD card
+
+   sudo dd if=u-boot.kpart of=/dev/sdc1; sync
+
+
+7. Start it up
+
+Reboot the device in dev mode. Make sure that you have USB booting enabled. To
+do this, login as root (via Ctrl-Alt-forward_arrow) and type
+'enable_dev_usb_boot'. You only need to do this once.
+
+Reboot the device with the SD card inserted. Press Clrl-U at the developer
+mode screen. It should show something like the following on the display:
+
+   U-Boot 2017.07-00637-g242eb42-dirty (May 22 2017 - 06:14:21 -0600)
+
+   Model: Acer Chromebook 13 CB5-311
+   Board: Google/NVIDIA Nyan-big, ID: 1
+
+   Net:   No ethernet found.
+   Hit any key to stop autoboot:  0
+   Tegra124 (Nyan-big) #
+
+
+8. Known problems
+
+On the serial console the word MMC is chopped at the start of the line:
+
+C:   sdhci@700b0000: 2, sdhci@700b0400: 1, sdhci@700b0600: 0
+
+This is likely due to some problem with change-over of the serial driver
+during relocation (or perhaps updating the clock setup in board_init()).
+
+
+9. Notes
+
+To check that you copied the u-boot.its file correctly, use these commands.
+You should see that the data at 0x100 in u-boot-chromium.fit is the first few
+bytes of U-Boot:
+
+   hd u-boot-chromium.fit |head -20
+   ...
+   00000100  b8 00 00 ea 14 f0 9f e5  14 f0 9f e5 14 f0 9f e5  |................|
+
+   hd b/nyan-big/u-boot.bin |head
+   00000000  b8 00 00 ea 14 f0 9f e5  14 f0 9f e5 14 f0 9f e5  |................|
+
+
+The 'data' property of the FIT is set up to start at offset 0x100 bytes into
+the file. The change to CONFIG_SYS_TEXT_BASE is also an offset of 0x100 bytes
+from the load address. If this changes, you either need to modify U-Boot to be
+fully relocatable, or expect it to hang.
+
+
+Other notes
+===========
+
+flashrom
+--------
+
+   Used to make a backup of your firmware, or to replace it.
+
+   See: https://www.chromium.org/chromium-os/packages/cros-flashrom
+
+
+coreboot
+--------
+
+Coreboot itself is not designed to actually boot an OS. Instead, a program
+called Depthcharge is used. This originally came out of U-Boot and was then
+heavily hacked and modified such that is is almost unrecognisable. It does
+include a very small part of the U-Boot command-line interface but is not
+usable as a general-purpose boot loader.
+
+In addition, it has a very unusual design in that it does not do device init
+itself, but instead relies on coreboot. This is similar to (in U-Boot) having
+a SPI driver with an empty probe() method, relying on whatever was set up
+beforehand. It can be quite hard to figure out between these two code bases
+what settings are actually used. When chain-loading into U-Boot we must be
+careful to reinit anything that U-Boot expects. If not, some peripherals (or
+the whole machine) may not work. This makes the process of chainloading more
+complicated than it could be on some platforms.
+
+Finally, it supports only a subset of the U-Boot's FIT format. In particular
+it uses a fixed address to load the FIT and does not support load/exec
+addresses. This means that U-Boot must be able to boot from whatever
+address Depthcharge happens to use (it is the CONFIG_KERNEL_START setting
+in Depthcharge). In practice this means that the data in the kernel@1 FIT node
+(see above) must start at the same address as U-Boot's CONFIG_SYS_TEXT_BASE.