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

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

Since commit 13b25489b6f8 ("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: 13b25489b6f8 ("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>