Refactor board_identifier configuration and internal storage

[Scottapotamas]

The board ID is currently a bit of a hack, using a 16-bit hash (and a poor hash at that) of whatever they pass in.

There are also warnings from stricter compilers if the developer follows our arduino example of inline providing a string and length, as passing a char* into a function should really force const char* instead.

Consider:

• Ability for real UUID's to be tracked - derived from a micro's UUID, MAC address or some SecureID chip.
• Ability for developer to feed in a garbage UUID - string data or etc.

Ideally the solution should aim to minimise impact (memory, flash) as much as possible without being impractical.