kbuild: replace two $(abs_objtree) with $(CURDIR) in top Makefile

Kbuild changes the working directory until it matches $(abs_objtree).

When $(need-sub-make) is empty, $(abs_objtree) is the same as $(CURDIR).

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>