bootstd: cros: Bring in some ChromiumOS structures

bootstd: cros: Bring in some ChromiumOS structures

Add a header file with structures for booting ChromiumOS, taken from the
vboot tree. Using these makes it easier to understand the code.

Note that the code style has not been updated for U-Boot, with use of
uint64_t,  __attribute__((packed)) and one comment-style nit. This should
make it easier to keep the code in sync. It was taken from commit:

   5b8596ce ("2sha256_arm: Fix data abort issue")

Update the CHROMEOS string to use the defined values.

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