esp32/Makefile: Include CFLAGS_EXTRA in CFLAGS definition.

Following other ports, so builds can be customised more easily, eg on the
command line building with a user C-module.

ports/esp32/Makefile

@@ -126,7 +126,7 @@ CFLAGS_COMMON = -Os -ffunction-sections -fdata-sections -fstrict-volatile-bitfie
 CFLAGS_BASE = -std=gnu99 $(CFLAGS_COMMON) -DMBEDTLS_CONFIG_FILE='"mbedtls/esp_config.h"' -DHAVE_CONFIG_H
 CFLAGS = $(CFLAGS_BASE) $(INC) $(INC_ESPCOMP)
 CFLAGS += -DIDF_VER=\"$(IDF_VER)\"
-CFLAGS += $(CFLAGS_MOD)
+CFLAGS += $(CFLAGS_MOD) $(CFLAGS_EXTRA)
 
 # this is what ESPIDF uses for c++ compilation
 CXXFLAGS = -std=gnu++11 $(CFLAGS_COMMON) $(INC) $(INC_ESPCOMP)