NRF52 low power file system corruption

[mikecarper]

Implement low power read only and very low power deep sleep

[BloodBlight]

Ran into this the other day, any attempt to transmit caused a reboot after a low power event. Backed up and restored to correct. Not sure if ATMOIC writes can be done, but that would negate the need.

[luigi311]

Should be solvable by the idea mentioned in #1370

[weebl2000]

See also #1601 to cleanu oprhan blobs still existing.