Add Settings Utility and example app

[TimJTi]

Summary

Last year @fjpanag offered to donate some code to the repo. This is my (second) attempt at making his "settings" functions NuttX compatible along with an example app to check it out

Impact

None. This is new.

Testing

On my custom board (SAMA5D27C-D1G).

There may still be some style and/or NuttX-convention errors/inconsistencies of course. Also, a reviewer previously reported that the dump_cache function was not safe to call from a signal handler but neither I nor the original author can understand why?

[fjpanag]

@TimJTi If you need to reduce the memory use of the map, why don't you try to use an RLE or a KVL serializer?

[TimJTi]

@TimJTi If you need to reduce the memory use of the map, why don't you try to use an RLE or a KVL serializer?

@fjpanag

See all answers above.

As far as this suggestion is concerned, it wouldn't fit very well with the existing storage_binary function which assumes every setting is the full sizeof settings_t? It would move the EEPROM storage outside of the entire settings feature whereas what I have done does fit in, and works, albeit needing some changes? If it is an overall objection to a third storage type that is, when all said and done, very close to storage_bin, could it perhaps be a Kconfig option to MINIMISE_BINSIZE, with the necessary differences included in storage_bin?

Also, to use KLV or RLE the encoding has to be written/ported/added to NuttX too? Is there an overarching reason to move the changes, that I feel are beneficial for EEPROM devices, outside of the settings feature itself and to "offload" them to the user app?

[TimJTi]

I have not been able to work for 6 weeks after emergency eye surgery and I have only just been able to revisit this.

I have made changes as per all comments and don't seem to have broken anything!