Add support for package-declared devices

Add a way for packages to declare files they need instead of relying
only on device tables, which creates files no matter if the package is
indeed enabled, as we can see for busybox.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>