build: Fix installation of target-dependent files

Pass all the relevant sub-directory make variables.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20140120112153.5685.30949.stgit@fimbulvetr.bsc.es

Makefile

@@ -399,7 +399,7 @@ endif
 		$(INSTALL_DATA) $(SRC_PATH)/pc-bios/keymaps/$$x "$(DESTDIR)$(qemu_datadir)/keymaps"; \
 	done
 	for d in $(TARGET_DIRS); do \
-	$(MAKE) -C $$d $@ || exit 1 ; \
+	$(MAKE) $(SUBDIR_MAKEFLAGS) TARGET_DIR=$$d/ -C $$d $@ || exit 1 ; \
         done
 
 # various test targets