kbuild: deb-pkg: fix build error with O=

Since commit 13b2548 ("kbuild: change working directory to external
module directory with M="), the Debian package build fails if a relative
path is specified with the O= option.

  $ make O=build bindeb-pkg
    [ snip ]
  dpkg-deb: building package 'linux-image-6.13.0-rc1' in '../linux-image-6.13.0-rc1_6.13.0-rc1-6_amd64.deb'.
  Rebuilding host programs with x86_64-linux-gnu-gcc...
  make[6]: Entering directory '/home/masahiro/linux/build'
  /home/masahiro/linux/Makefile:190: *** specified kernel directory "build" does not exist.  Stop.

This occurs because the sub_make_done flag is cleared, even though the
working directory is already in the output directory.

Passing KBUILD_OUTPUT=. resolves the issue.

Fixes: 13b2548 ("kbuild: change working directory to external module directory with M=")
Reported-by: Charlie Jenkins <charlie@rivosinc.com>
Closes: https://lore.kernel.org/all/Z1DnP-GJcfseyrM3@ghost/
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: masahir0y

scripts/package/install-extmod-build

@@ -69,7 +69,7 @@ if [ "${CC}" != "${HOSTCC}" ]; then
 	#
 	# Use the single-target build to avoid the modpost invocation, which
 	# would overwrite Module.symvers.
-	"${MAKE}" HOSTCC="${CC}" KBUILD_EXTMOD="${destdir}" scripts/
+	"${MAKE}" HOSTCC="${CC}" KBUILD_OUTPUT=. KBUILD_EXTMOD="${destdir}" scripts/
 
 	cat <<-'EOF' >  "${destdir}/scripts/Kbuild"
 	subdir-y := basic
@@ -78,7 +78,7 @@ if [ "${CC}" != "${HOSTCC}" ]; then
 	EOF
 
 	# Run once again to rebuild scripts/basic/ and scripts/mod/modpost.
-	"${MAKE}" HOSTCC="${CC}" KBUILD_EXTMOD="${destdir}" scripts/
+	"${MAKE}" HOSTCC="${CC}" KBUILD_OUTPUT=. KBUILD_EXTMOD="${destdir}" scripts/
 
 	rm -f "${destdir}/Kbuild" "${destdir}/scripts/Kbuild"
 fi