This will allow initrds to omit certain files, such as other initrds,
irrelevant files, iles for another platform, and so on. This will be useful
when initrd contain entire system roots.
This enables useful features such as directories, CRC32 checksums, and other
useful features. The initrdfs in the kernel is now hooked up against the new
API, although the kernel's current limited FS support is a problem for now.
To work around that, directories are not supported at runtime, although the
internal API understands them wonderfully. This will be fixed when the
kernel gets a real VFS.
