kbuild: rename abs_objtree to abs_output

'objtree' refers to the top of the output directory of kernel builds.

Rename abs_objtree to a more generic name, to better reflect its use in
the context of external module builds.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>