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/README.md

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+# mkspiffs Create spiffs image
+
+mkspiffs is a command line utility to create an image of SPIFFS in order
+to write to flash.
+
+## Usage
+
+mkspiffs will be built automatically if you include the following line in your
+makefile:
+
+```
+$(eval $(call make_spiffs_image,files))
+```
+
+where *files* is the directory with files that should go into SPIFFS image.
+
+Or you can build mkspiffs manually with:
+
+```
+make SPIFFS_SIZE=0x100000
+```
+
+mkspiffs cannot be built without specifying SPIFFS size because it uses the
+same SPIFFS sources as the firmware. And for the firmware SPIFFS size is
+compile time defined.
+
+Please note that if you change SPIFFS_SIZE you need to rebuild mkspiffs.
+The easiest way is to run `make clean` for you project.
+
+To manually generate SPIFFS image from directory, run:
+
+```
+mkspiffs DIRECTORY IMAGE_NAME
+```