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>
diff --git a/.gitignore b/.gitignore
index 4e4fd00..2ddf57f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -49,8 +49,7 @@
 /errlog
 /reloc_off
 
-/spl/*
-!/spl/Makefile
+/spl/
 /tpl/
 
 #