Makefile.hw: avoid overly large 'make clean' rm command

Avoid 'make clean' producing an 'rm' command which has a lot
of duplicate 'hw//*.o' arguments, by using $(sort $(dir ..))
rather than $(dir $(sort ..)) so Make's sort function will
remove the duplicates for us. We can also remove the double
'//' safely because $(dir ..) is guaranteed to return a string
ending in '/'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Makefile.hw

@@ -19,8 +19,8 @@ all: $(hw-obj-y)
 	@true
 
 clean:
-	rm -f $(addsuffix /*.o, $(dir $(sort $(hw-obj-y))))
-	rm -f $(addsuffix /*.d, $(dir $(sort $(hw-obj-y))))
+	rm -f $(addsuffix *.o, $(sort $(dir $(hw-obj-y))))
+	rm -f $(addsuffix *.d, $(sort $(dir $(hw-obj-y))))
 
 # Include automatically generated dependency files
 -include $(patsubst %.o, %.d, $(hw-obj-y))