x86: Correct Minnowboard instructions to use the right descriptor

The descriptor provided with the FSP does not seem to work. Update the
instructions to use the descriptor from the original Intel firmware.

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

doc/README.x86

@@ -127,15 +127,32 @@ board/intel/minnowmax/fsp.bin
 Obtain the VGA RAM (Vga.dat at the time of writing) and put it into the same
 directory: board/intel/minnowmax/vga.bin
 
-You still need two more binary blobs. These come from the sample SPI image
-provided in the FSP (SPI.bin at the time of writing).
+You still need two more binary blobs. The first comes from the original
+firmware image available from:
+
+http://firmware.intel.com/sites/default/files/2014-WW42.4-MinnowBoardMax.73-64-bit.bin_Release.zip
+
+Unzip it:
+
+   $ unzip 2014-WW42.4-MinnowBoardMax.73-64-bit.bin_Release.zip
 
 Use ifdtool in the U-Boot tools directory to extract the images from that
 file, for example:
 
+   $ ./tools/ifdtool -x MNW2MAX1.X64.0073.R02.1409160934.bin
+
+This will provide the descriptor file - copy this into the correct place:
+
+   $ cp flashregion_0_flashdescriptor.bin board/intel/minnowmax/descriptor.bin
+
+Then do the same with the sample SPI image provided in the FSP (SPI.bin at
+the time of writing) to obtain the last image. Note that this will also
+produce a flash descriptor file, but it does not seem to work, probably
+because it is not designed for the Minnowmax. That is why you need to get
+the flash descriptor from the original firmware as above.
+
    $ ./tools/ifdtool -x BayleyBay/SPI.bin
    $ cp flashregion_2_intel_me.bin board/intel/minnowmax/me.bin
-   $ cp flashregion_0_flashdescriptor.bin board/intel/minnowmax/descriptor.bin
 
 Now you can build U-Boot and obtain u-boot.rom