Fixup nandroid-md5.sh with legitimate return code

The reason md5sum would return an error status code is because it was
being executed on . and .. paths. Only run md5sum on files and allow the
script to exit with an error status if md5sum fails.

Change-Id: Ieeaa5e65ab3cf6b6470815c3ec13cd3a8c8b6b5e