kbuild: modules-cpio-pkg: Respect INSTALL_MOD_PATH

The modules-cpio-pkg target added in commit 2a9c8c0 ("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/ [1]
Fixes: 2a9c8c0 ("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>

Author: jannau

scripts/Makefile.package

@@ -195,7 +195,7 @@ tar%-pkg: linux-$(KERNELRELEASE)-$(ARCH).tar.% FORCE
 .tmp_modules_cpio: FORCE
 	$(Q)$(MAKE) -f $(srctree)/Makefile
 	$(Q)rm -rf $@
-	$(Q)$(MAKE) -f $(srctree)/Makefile INSTALL_MOD_PATH=$@ modules_install
+	$(Q)$(MAKE) -f $(srctree)/Makefile INSTALL_MOD_PATH=$@/$(INSTALL_MOD_PATH) modules_install
 
 quiet_cmd_cpio = CPIO    $@
       cmd_cpio = $(CONFIG_SHELL) $(srctree)/usr/gen_initramfs.sh -o $@ $<
@@ -264,6 +264,7 @@ help:
 	@echo '  tarxz-pkg           - Build the kernel as a xz compressed tarball'
 	@echo '  tarzst-pkg          - Build the kernel as a zstd compressed tarball'
 	@echo '  modules-cpio-pkg    - Build the kernel modules as cpio archive'
+	@echo '                        (uses INSTALL_MOD_PATH inside the archive)'
 	@echo '  perf-tar-src-pkg    - Build the perf source tarball with no compression'
 	@echo '  perf-targz-src-pkg  - Build the perf source tarball with gzip compression'
 	@echo '  perf-tarbz2-src-pkg - Build the perf source tarball with bz2 compression'