udoo: Add a README file

Add a README file to explain how to build and flash the SD card
for Udoo boards.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>

board/udoo/README

+How to use U-Boot on MX6Q/DL Udoo boards
+----------------------------------------
+
+- Build U-Boot for MX6Q/DL Udoo boards:
+
+$ make mrproper
+$ make udoo_defconfig
+$ make
+
+This will generate the SPL image called SPL and the u-boot.img.
+
+- Flash the SPL image into the SD card:
+
+sudo dd if=SPL of=/dev/mmcblk0 bs=1k seek=1; sync
+
+- Flash the u-boot.img image into the SD card:
+
+sudo dd if=u-boot.img of=/dev/mmcblk0 bs=1k seek=69; sync
+
+- Insert the SD card in the board, power it up and U-Boot messages should
+come up.