Support mailbox ID definition at runtime

[denis-stepanov]

Currently mailbox ID is compiled in on the sender side. The idea would be for a sender to provide some unique ID instead (probably derived from ESP MAC) and then allow remapping this on the receiver side to an ID=1,2 etc.

To get a reasonably unique ID one would likely need to modify the protocol to send a long string, which would not only be incompatible change, but could easily double the packet size. So the feature to be taken with caution.

[denis-stepanov]

Another option would be to use a separate sketch to ask the ID from the user and write it to EEPROM. The sketch would need to be run once after module assembly. This would not require modification of the protocol. In absence of configuration a digit from ESP MAC could be taken, which would give some - poor, but not zero - probability to distinguish the units by default. The sketch could be potentially combined with the sketch used for battery calibration (see feature #47).