serial_bt: implement the serial over Bluetooth transport (conn=bt/...)

serial_bt: implement the serial over Bluetooth transport (conn=bt/...)

Introduce the serial_bt.c source file which implements the methods of a
serial transport and calls into the platform agnostic src/bt/ support
code.

Implement support for several chips and modules: RFCOMM (BT classic,
tested with HC-05), BLE122 (tested with 121GW), Nordic nRF51, and TI
CC254x (the latter untested). Read support is assumed to be complete,
write support for BLE may be incomplete due to lack of access to
hardware for tests.