Don't use relative imports like `../foo.h` as those depend on the
relative position of both files. Rather than that use imports relative
to the `src` directory, which explicitly is part of the include
directories.
* add submodule littlefs
* base fs
* Save settings using littlefs
* Small fixes and suggestions from PR
* More small fixes from PR suggestions
* Code clean up
* Change SpiNorFlash functions to be private in FS