image: make images and artifacts dependent of initramfs

image: make images and artifacts dependent of initramfs

There is currently a BIG bug in how the images dependency is handled and
recent Per Device Rootfs made this more clear and less statistical.

There is currently no dependency between images/artifacts build with
initramfs build. This cause whatever additional image that depends on an
initramfs image to fail as it might happen that image and initramfs
build are called at the same time and the additional image is called
before initramfs build has finished.

Each image-command assume the source image to be taken from the /bin
directory but that is only copied from the /tmp directory only at the
end of the process.

Artifacts currently depends on image with the use of the
BOARD-NAME-images Makefile target, but this is not the case for
initramfs that also define a -images Makefile target but that is not
accounted in images (that might depend on some initramfs images)

To actually fix this, introduce a new Makefile target, -initramfs-images
and make image and artifacts build to depend on this. Since initramfs
images are optional, this dependency is actived only when initramfs
image are built.

With this change we correctly enforce the build order:
- Initramfs Images (optional)
- Images
- Artifacts

(cherry picked from commit cc6a0abcabf718df872c0f6876d1121d3dd09953)
[ rebased on openwrt-23.05 ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>