build common .o files everywhere (fixes issue #29)

vector.o, nvic.o, scb.o and assert.o are available on every platform,
but at least some of them differ between the implementations. they
already got built explicityly on some platforms; now adding them to the
common Makefile.include.

lib/Makefile.include

@@ -23,6 +23,9 @@ ifneq ($(V),1)
 Q := @
 endif
 
+# common objects
+OBJS += vector.o systick.o scb.o nvic.o assert.o
+
 all: $(SRCLIBDIR)/$(LIBNAME).a
 
 $(SRCLIBDIR)/$(LIBNAME).a: $(SRCLIBDIR)/$(LIBNAME).ld $(OBJS)