make it possible to do 'make -C sw/' even with builddir!=srcdir

The change in partial_build.mk assumes all the Makefile's using it
are in builddir/<module>/ or builddir/external/<module> , these are
differentiated by checking for ../External_module.mk .

Change-Id: Iddc8fa2ec0842f181780f7491cf5a2244efd014a