Makefile: abolish COBJS, SOBJS, etc.

The support for COBJS, COBJS-y, SOBJS, SOBJS-y, GLCOBJS, GLSOBJS
from scripts/Makefile.build.
Going forward we need to use Kbuild style consistently.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>