buildroot

fs: new intermediate rootfs-common to gather common dependencies

Before we can create an intermediate tarball for all filesystems, we
nedd to move the common dependencies needed to generate that
intermediate tarball, rather than leave those dependencies to each
filesystem.

So, we introduce rootfs-common, which gathers all those common
dependencies.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>