The bootloader backup is intentionally simple, but it's a little
*too* simple. Add a sanity check to make sure what we're backing
up or restoring contains the first 8 bytes of the SPL header.
This isn't going to catch all possible problems, but it'll stop
obviously non-functional backups from being restored.
Change-Id: I6e80351aeb96c467f0514bd0ecd77d94ff72a8f8