system: Fix warning when selecting systemd

With systemd and a custom skeleton, you end up with the following
warning:
warning: (BR2_INIT_SYSTEMD) selects BR2_ROOTFS_MERGED_USR which has
unmet direct dependencies (BR2_ROOTFS_SKELETON_DEFAULT)

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