kbuild: move spl/Makefile to scripts/Makefile.spl

All files under spl/ and tpl/ are generated during the build process
except spl/Makefile.

We can simplify clean-rule and git-ignore by moving spl/Makefile
to somewhere else.

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