Implement BLE purge techniques

[chris-bc]

Configurable behaviour on BLE out of memory when adding a new device:

• Truncate all non-selected BT devices and continue
• Truncate all unnamed BT devices and continue
• Truncate unnamed and unselected BT devices and continue
• Truncate the oldest BT devices and continue
• Truncate lowest RSSI (could even write an incremental cutoff)

• Multiple purge strategies may be selected
• Priority of application is RSSI, Age, Selected, Named
• Minimum purgeable RSSI and minimum purgeable Age are configurable from menuconfig or as command-line arguments.

[chris-bc]

All purge strategies implemented, not yet tested (commits are pending push).

[chris-bc]

OK, I'm only going to be able to test this on the train or in the office. That's a pain, but it'll get there.

[chris-bc]

Mostly worked first attempt - Age turned out to be a bit messy though