Create new cros_write_firmware tool based on write_tegra_bios.

This tool uses U-Boot instead of the binary bootloader.bin to
do the actual flashing of the boot device.  This gives us more
control over the firmware flashing process.  In particular,
U-Boot has SPI support that we can customize, which allows
us to write a chromium firmware image directly to SPI.

TEST=Build and boot from SPI U-Boot/kernel/ChromiumOS on Kaen

Change-Id: I37f6b3d00b23bec0e79da2282b2f0e75fce76900

This CL depends on:

Review URL:
3 files changed