Store md5sum of all files being available in build output directory

Store md5sum of all files being available in build output directory

As of commit ff80c12bbdaf7ffaff4722bbd96808f2dbfbcad9
['Install "GRML" files into GRML/$GRML_NAME/']
the squashfs file is missing in the md5sum file, even
though the squashfs file is the most important one to
check for.