optionrom: cope with multiple -O options

Reproducer:

    CFLAGS="-g3 -O0" ./configure --target-list=aarch64-softmmu,arm-softmmu --enable-vhost-net --enable-virtfs

Here CFLAGS ends up with "-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 ... -g3 -O0"
and pc-bios/optionrom/Makefile forgets to add the -O2 it needs.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

pc-bios/optionrom/Makefile

@@ -10,10 +10,7 @@ $(call set-vpath, $(SRC_PATH)/pc-bios/optionrom)
 .PHONY : all clean build-all
 
 # Compiling with no optimization creates ROMs that are too large
-ifeq ($(filter -O%, $(CFLAGS)),)
-override CFLAGS += -O2
-endif
-ifeq ($(filter -O%, $(CFLAGS)),-O0)
+ifeq ($(lastword $(filter -O%, -O0 $(CFLAGS))),-O0)
 override CFLAGS += -O2
 endif