kbuild: add target to build a cpio containing modules

kbuild: add target to build a cpio containing modules

Add a new package target to build a cpio archive containing the kernel
modules. This is particularly useful to supplement an existing initramfs
with the kernel modules so that the root filesystem can be started with
all needed kernel modules without modifying it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Co-developed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20251125-cpio-modules-pkg-v2-2-aa8277d89682@pengutronix.de
Signed-off-by: Nicolas Schier <nsc@kernel.org>