kbuild: install-extmod-build: Fix build when specifying KBUILD_OUTPUT

Since commit 5f73e7d ("kbuild: refactor cross-compiling
linux-headers package"), the linux-headers pacman package fails
to build when "O=" is set. The build system complains:

/mnt/chroot/linux/scripts/Makefile.build:41: mnt/chroots/linux-mainline/pacman/linux-upstream/pkg/linux-upstream-headers/usr//lib/modules/6.14.0-rc3-00350-g771dba31fffc/build/scripts/Makefile: No such file or directory

This is because the "srcroot" variable is set to "." and the
"build" variable is set to the absolute path. This makes the
"src" variables point to wrong directory.

Change the "build" variable to a relative path to "." to
fix build.

Fixes: 5f73e7d ("kbuild: refactor cross-compiling linux-headers package")
Signed-off-by: Inochi Amaoto <inochiama@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: inochisa

scripts/package/install-extmod-build

@@ -63,7 +63,7 @@ if [ "${CC}" != "${HOSTCC}" ]; then
 	# Clear VPATH and srcroot because the source files reside in the output
 	# directory.
 	# shellcheck disable=SC2016 # $(MAKE) and $(build) will be expanded by Make
-	"${MAKE}" run-command KBUILD_RUN_COMMAND='+$(MAKE) HOSTCC='"${CC}"' VPATH= srcroot=. $(build)='"${destdir}"/scripts
+	"${MAKE}" run-command KBUILD_RUN_COMMAND='+$(MAKE) HOSTCC='"${CC}"' VPATH= srcroot=. $(build)='"$(realpath --relative-base=. "${destdir}")"/scripts
 
 	rm -f "${destdir}/scripts/Kbuild"
 fi