kbuild: generate {spl, tpl}-autoconf.mk only when it is necessary

Before this commit, {spl,tpl}-autoconf.mk was always generated
at the top Makefile even if SPL(TPL) build was not selected.

This commit moves the build rule of {spl,tpl}-autoconf.mk
from the top Makefile to spl/Makefile.
It prevents unnecessary {spl,tpl}-autoconf.mk from being
generated.

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