fs: introduce variable with all recursive dependencies

fs: introduce variable with all recursive dependencies

This allows getting all the recursive dependencies of filesystems,
ike we have for packages, and allows us to treat both in a similar
fashion.

Reported-by: Arnout Vandecappelle <[email protected]>
Signed-off-by: "Yann E. MORIN" <[email protected]>
Cc: Arnout Vandecappelle <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Cc: Thomas De Schampheleire <[email protected]>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>