kbuild: rename OBJTREE to objtree

Prior to Kbuild, $(OBJTREE) was used for pointing to the
top of build directory with absolute path.

In Kbuild style, $(objtree) is used instead.
This commit renames OBJTREE to objtree and delete the
defition of OBJTREE.

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