brcm2708: Add feature flag rootfs-part

brcm2708: Add feature flag rootfs-part

Even with squashfs brcm2708 requires ROOTFS_PART_SIZE because the overlay
exists as a loopback device on the space not used by squashfs in the root
partition. Also for ext4 (the other fs option) ROOTFS_PART_SIZE is required,
so use feature flag rootfs-part to enable it.

Fixes FS#2166

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>
(cherry picked from commit 3bb44f42990a75e66972016cde75bed6a3f09ef9)
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>