image: respect TARGET_PER_DEVICE_ROOTFS for initramfs

image: respect TARGET_PER_DEVICE_ROOTFS for initramfs

Initramfs images were using a common rootfs (TARGET_DIR) for all
devices, ignoring TARGET_PER_DEVICE_ROOTFS. If a single device required
a package to build a functional initramfs image, it should be included
by default for all devices or that device should be isolated into a new
subtarget. Now the initramfs will be built using the target-specific

Implementing Per Device Rootfs for Initramfs is not trivial as the
rootfs needs to be embedded in the kernel image. The kernel supports an
option to define the initramfs location and the image generation for the
kernel can't be run in parallel as other checks are done to config and
other arch dependent files.

To handle this, we prepare a config for each rootfs and we generate the
images under lock to prevent problem with parallel execution.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
[ rework implementation for locking support ]
Link: https://github.com/openwrt/openwrt/pull/12959
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>