push LOAD_ADDR out to arch mk files

Rather than maintain/extend the current ifeq($(ARCH)) mess that exists in
the standalone Makefile, push the setting up of LOAD_ADDR out to the arch
config.mk (and rename to STANDALONE_LOAD_ADDR in the process).  This keeps
the common code clean and lets the arch do whatever crazy crap it wants in
its own area.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>