Create SPIFFS image and flash it.

mkspiffs is added to create SPIFFS image from directory with files.
Build process changed to flash SPIFFS image if necessary

extras/spiffs/mkspiffs/Makefile

@@ -0,0 +1,37 @@
+# Check if SPIFFS_SIZE defined only if not cleaning
+ifneq ($(MAKECMDGOALS),clean)
+ifndef SPIFFS_SIZE
+define ERROR_MSG
+Variable SPIFFS_SIZE is not defined.
+Cannot build mkspiffs without SPIFFS_SIZE.
+Please specify it in your application Makefile.
+
+endef
+$(error $(ERROR_MSG))	
+endif
+endif
+
+SOURCES := spiffs_hydrogen.c
+SOURCES += spiffs_cache.c
+SOURCES += spiffs_gc.c
+SOURCES += spiffs_check.c
+SOURCES += spiffs_nucleus.c
+SOURCES += mkspiffs.c
+
+OBJECTS := $(SOURCES:.c=.o)
+
+VPATH = ../spiffs/src
+
+CFLAGS += -I..
+CFLAGS += -DSPIFFS_BASE_ADDR=0   # for image base addr is start of the image
+CFLAGS += -DSPIFFS_SIZE=$(SPIFFS_SIZE)
+
+all: mkspiffs
+
+mkspiffs: $(OBJECTS)
+
+clean:
+	@rm -f mkspiffs
+	@rm -f *.o
+
+.PHONY: all clean