Currently, for targets that use the CONFIG_TARGET_ROOTFS_TARGZ a single
rootfs tarball is generated for the subtarget based of $(TARGET_DIR).
However, this means that it does not respect DEVICE_PACKAGES like other
rootfs images.
So, lets augment CONFIG_TARGET_ROOTFS_TARGZ by adding a proper targz fstype
so that per device rootfs is generated under lock.
This is required so that devices that use custom sysupgrade archives like
Methode devices, can actually include a per device rootfs so when building
for multiple devices and with CONFIG_TARGET_PER_DEVICE_ROOTFS set the built
image actually includes the listed DEVICE_PACKAGES.
Signed-off-by: Robert Marko <robert.marko@sartura.hr>