Add back rootfs-* dependencies to "source" and "legal-info" targets

After adding support top-level parallel make the rootfs-* dependencies
were not anymore considered for the "source" and "legal-info" targets
because the rootfs-* targets were removed from TARGETS variable and
placed in the TARGETS_ROOTFS variable so to fix the issue use use both
"TARGETS" and "TARGETS_ROOTFS" variables.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Vincent Stehlé <vincent.stehle@freescale.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>