LIBUART: Do not use built-in archive rule

Do not use the built-in archive creation rule of
make because it is not parallel-save. Even the manual
states this, however, in some rather cryptic way.
But the manual is actually right.

Change-Id: I6542bc3e28a5155868593b59083bbe7ccb3ab319

kernel/fiasco/src/lib/uart/Makefile

@@ -3,10 +3,11 @@ include $(srcdir)/Makerules.global
 
 vpath %.cc $(VPATH_LIBUART)
 
-all: $(notdir $(TARGET))($(OBJECTS))
+all: $(notdir $(TARGET))
 
 $(OBJECTS): $(objbase)/.Compiler-config
 
 $(notdir $(TARGET)): $(OBJECTS)
+	$(AR) $(ARFLAGS) $@ $+
 
 -include $(wildcard .*.d)