fs: only add non rootfs- targets to PACKAGES

Commit 4570dbcb
("4570dbcb") added the dependencies of
rootfs building logic to PACKAGES, only thinking about the case where
rootfs building logic actually depends on real, normal packages
(host-mtd, host-genext2fs, etc.). But some file systems (rootfs-ubi)
depend on other filesystem images (rootfs-ubifs). And such targets
should not be added to PACKAGES, otherwise an incorrect circular
dependency is created.

This commit fixes that by only adding the rootfs building logic
dependencies that do *not* start with rootfs- to the global PACKAGES
variable.

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