optionrom: remove use of implicit RM variable

Signed-off-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

pc-bios/optionrom/Makefile

@@ -24,4 +24,4 @@ build-all: multiboot.bin
 	$(call quiet-command,$(SHELL) $(SRC_PATH)/pc-bios/optionrom/signrom.sh $< $@,"  Signing $(TARGET_DIR)$@")
 
 clean:
-	$(RM) *.o *.d *.raw *.img *.bin *~
+	rm -f *.o *.d *.raw *.img *.bin *~