Makefile: allow boards to check file size limits

Boards often have a reserved size limit on the flash where they're stored.
Sometimes during upgrades or config changes, those limits are exceeded,
but no one notices until they try to upgrade and the limit screws things
up.  Either not enough of U-Boot is written to flash (and so the reboot
fails), or too much is written (and so things after it get clobbered).

So allow boards to declare a size limit (in bytes) and have the build
system check it while building.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>