Log status menu

Implements issue #56 : add a submenu to display status of log storage on the device. Fairly complete implementation, you can find this in the "Advanced" menu, displays percentage of log full, how much time is left for logging at the current log interval, as well as number of log records used in flash out of the total number of available records (the latter is more technical, but will be interesting for advanced users).

Would be worth also adding serial commands to get log status as well.

This bumps release to 12.17-devel, by the way.