fs: make it behave a bit more like the package infra

fs: make it behave a bit more like the package infra

Currently, to register a filesystem, one has to call:

    $(eval $(call ROOTFS_TARGET,blabla))

This is very unlike the package infrastructure, where the name of the
package is automatically guessed by the infra.

It turns out that we can now do that for the filesystem infra too.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>