kbuild: pkg: rename scripts/package/Makefile to scripts/Makefile.package

scripts/package/Makefile does not use $(obj) or $(src) at all.
It actually generates files and directories in the top of $(objtree).
I do not see much sense in descending into scripts/package/.

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