SPIFFS submodule and fixed build for both platforms

[me-no-dev]

• use submodule instead of copies of the SPIFFS source
• move config to it's own folder
• update Makefile to work with the new file structure
• fix build defines for ESP8266 and ESP32 (tested with OSX on both targets)

[me-no-dev]

@igrr this is good to go :)