include: android_bootloader_message.h: Minimize the diff to AOSP

include: android_bootloader_message.h: Minimize the diff to AOSP

Perform the following updates:
 - Relocate the commit id from the file to the description of U-Boot
   commit. The AOSP commit is c784ce50e8c10eaf70e1f97e24e8324aef45faf5.
   This is done to avoid stale references in the file itself. The
   reasoning is in https://patchwork.ozlabs.org/patch/1098056/#2170209.
 - Minimize the diff to AOSP, to decrease the effort of the next AOSP
   backports. The background can be found in:
   https://patchwork.ozlabs.org/patch/1080394/#2168454.
 - Guard the static_assert() calls by #ifndef __UBOOT__ ... #endif,
   to avoid compilation failures of files including the header.

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>