esp8266: Support CFLAGS_EXTRA.

esp8266/Makefile

@@ -31,7 +31,7 @@ CFLAGS_XTENSA = -fsingle-precision-constant -Wdouble-promotion \
 	-Wl,-EL -mlongcalls -mtext-section-literals \
 
 CFLAGS = $(INC) -Wall -Wpointer-arith -Werror -ansi -std=gnu99 -nostdlib -DUART_OS=$(UART_OS) \
-	$(CFLAGS_XTENSA) $(COPT)
+	$(CFLAGS_XTENSA) $(COPT) $(CFLAGS_EXTRA)
 
 LDFLAGS = -nostdlib -T esp8266.ld -Map=$(@:.elf=.map) --cref
 LIBS = -L$(ESP_SDK)/lib -lmain -ljson -llwip -lpp -lnet80211 -lwpa -lphy -lnet80211