Add API to get temperature value of SX 127x chip

[cyberman54]

The semtech SX1276/77/78/79 chip, which LMIC is focused on, delivers a temperature value in register 0x3c. This gives the option to create some added value with LORAWAN nodes by estimating the surrounding temperature, maybe doing some edge computing with this values and generating payload from it, e.g. sending temperature threshold alarms to a backend.

Thus, we need an API to retrieve the temperature value from LMIC and an appropriate enhancement in radio.c to query the register value from the SX127x chip.

[cyberman54]

Demo PR see #774