FASTBOOT: Use BUILD_TAG as the version if it's defined

BUILD_TAG can be passed on the make command line so it's a good
way for the build server to set the version without having to touch
a file all the time.  Fallback to use CONFIG_FASTBOOT_VERSION_BOOTLOADER
if it's defined, or FASTBOOT_VERSION if not.

Change-Id: Ib88a0d30f6e049040e978978f6e4ed452499ff12
Signed-off-by: Mike J. Chen <mjchen@google.com>
1 file changed