All build and package functions share the following commands:

  export MAKEFLAGS="${KBUILD_MAKEFLAGS}"
  cd "${objtree}"

Factor out the common code.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by:   Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Christian Heusel <christian@heusel.eu>