kbuild: mergeconfig: remove redundant $(objtree)

Kbuild always runs in $(objtree).  Actually, $(objtree) is always
set to "." by the top-level Makefile.

We can omit "-O $(objtree)" and "$(objtree)/".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>