image: respect TARGET_PER_DEVICE_ROOTFS for initramfs fit command

image: respect TARGET_PER_DEVICE_ROOTFS for initramfs fit command

Fit command makes use of CONFIG_TARGET_ROOTFS_INITRAMFS_SEPARATE as the
cpio is provided externally and is not embedded in the kernel image.

As done with embedded cpio, also handle PER_DEVICE_ROOTFS by generating
a cpio for each rootfs and reference them by the ROOTFS_ID generated
previously. The generated cpio are placed in the linux directory + the
package ID.

Link: https://github.com/openwrt/openwrt/pull/12959
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>