kbuild: modules-cpio-pkg: Respect INSTALL_MOD_PATH

kbuild: modules-cpio-pkg: Respect INSTALL_MOD_PATH

The modules-cpio-pkg target added in commit 2a9c8c0b59d3 ("kbuild: add
target to build a cpio containing modules") is incompatible with
initramfs with merged /lib and /usr/lib directories [1]. "/lib" cannot
be a link and directory at the same time.
Respect a non-empty INSTALL_MOD_PATH in the modules-cpio-pkg target so
that `make INSTALL_MOD_PATH=/usr modules-cpio-pkg` results in the same
module install location as `make INSTALL_MOD_PATH=/usr modules_install`.

Tested with Fedora distribution initramfs produced by dracut.

Link: https://systemd.io/THE_CASE_FOR_THE_USR_MERGE/
Fixes: 2a9c8c0b59d3 ("kbuild: add target to build a cpio containing modules")
Cc: stable@vger.kernel.org
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Tested-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://patch.msgid.link/20260327-kbuild-modules-cpio-pkg-usr-merge-v3-1-ef507dfa006c@jannau.net
Signed-off-by: Nathan Chancellor <nathan@kernel.org>