build/firmware/coreboot: confirm compiled roms

return with error status if no images were compiled

if a rom image fails to compile, then it will also
exit with error status, but sometimes you can pass
argument "cros" or "default", and it would not give
you rom images due to no target.cfg files, but these
are also ignored because of that.

this restores the same behaviour that existed before,
for this final error check.

Signed-off-by: Leah Rowe <leah@libreboot.org>

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