This commit avoids processing C=... and M=... twice
when O=... is also given.

Besides, we can also remove KBUILD_EXTMOD="$(KBUILD_EXTMOD)"
in the sub-make target.
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: NPeter Foley <pefoley2@pefoley.com>
Signed-off-by: NMichal Marek <mmarek@suse.cz>