libpayload: Fix immediate rebuild after a clean

libpayload: Fix immediate rebuild after a clean

After running make clean, most of build/ directory gets deleted.
It is (re)created in the "prepare" make target, but that was libpayload.a
dependency after the $OBJS, while OBJS building already needs to dump its
created object files there.
Simply rearrange the make target dependencies to get at least "make clean;make" working.

Signed-off-by: Mart Raudsepp <mart.raudsepp@artecdesign.ee>
Acked-by: Jordan Crouse <jordan@cosmicpenguin.net>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@3771 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1