fs/common.mk: Fix show-build-order

The command "make show-build-order" doesn't show dependencies of rootfs-common target.

This patch adds $(ROOTFS_COMMON_DEPENDENCIES) to PACKAGES variable.

Signed-off-by: Serj Kalichev <serj.kalichev@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>