The mkj6romfs.py script always wrote uncompessed directory info to the
initrd image, even if compressed was smaller - but it would write the
metadata as if it had compressed it.
CDB seemed to be too simple for the needs of init, and squashfs is too
laden with design choices to work around Linux's APIs. This commit adds
creation of an initrd image of a new format I've called `j6romfs`.
Note that this commit currently does not work! The initrd-reading code
still needs to be added.
