microbit: Add explicit service changed notification
microbit: Add explicit service changed notification

BLE central devices (such as mobile phones/tablets) are permitted to cache the
list of services and characteristics provided by a BLE peripheral such as the
micro:bit. This was causing compatibility problems with IOS during FOTA
flashing, as the the list of BLE services may change after flashing.

markshannon/microbit-dal,markshannon/microbit-dal