config: no need to mix arch makefiles

Move the config/config-*.mak files to their own directories and
rename to Makefile.* README is also updated to reflect the change,
as well as to remove an optimistic reference to a non-existent
docs directory.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>