fs/cpio: new option to use dracut tool

fs/cpio: new option to use dracut tool

Adds an option to invoke the dracut host tool, providing
a configuration file, instead of having a full cpio archive
of the whole target directory.

A default configuration file is provided. It assumes busybox init.
ideally, a different default should be used in case of systemd init, but
that's left for future work.

Signed-off-by: Thierry Bultel <[email protected]>
[[email protected]: many changes]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>
[[email protected]: some additional fixups]
Signed-off-by: Yann E. MORIN <[email protected]>