kbuild: get rid of $(realpath ...) from scripts/mkmakefile

Both relative path and absolute path have pros and cons. For example,
we can move the source and objtree around together by using the
relative path to the source tree.

Do not force the absolute path to the source tree. If you prefer the
absolute path, you can specify KBUILD_ABS_SRCTREE=1.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>