kbuild: parse C= and M= before changing the working directory

If Kbuild recurses to the top Makefile (for example, 'make deb-pkg'),
C= and M= are parsed over again, needlessly.

Parse them before changing the working directory. After that,
sub_make_done is set to 1, so they are parsed just once.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>