kernel: rework Initramfs compile cpio handling

kernel: rework Initramfs compile cpio handling

Now that we copy and then delete the Per-Device rootfs linux directory,
it's not valid anymore placing the generated cpio there as artifacts
or subsequent commands need the generated cpio.

To handle this, rework Initramfs compile cpio handling by placing them in
the KERNEL_BUILD_DIR but add to the name the rootfs HASH ID.

To also prevent race condition, generate and access these file under a
lock to prevent fit command to reference a cpio while a parallel
execution is genereting it.

Fixes: 52cc9d82f113 ("kernel: rework Initramfs locking logic")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>