Super-Cap Safe Shutdown software-side missing

[gardners]

As mentioned in #1, we need to implement safe shutdown functionality, whenever a Mesh Extender
believes it is running from the super-capacitor.

Instrumenting servald and lbard to not write to any files when GPIO24 is low is probably the simplest first step in this direction. This also solves the problem of ensuring that if power is restored before the device shuts down, that services can then resume.