Add code for internal temperature sensor

[PatrickKa]

The STM32 has an internal analog temperature sensor that is connected to the ADC. For more details see the datasheet sections 3.30 Temperature sensor and 6.3.21 Temperature sensor characteristics as well as the reference manual RM0383 section 11.9 Temperature sensor.

Reading the internal sensor should be similar to the RF temperature sensor.

[M-C-machts]

Notes:
our STM32 has only one ADC channel,
MCU temperature is on internal pin 16 (and not 18 like the manual suggests...)

[M-C-machts]

blocked by: issue is refusing to work for a long time

[PatrickKa]

That's not what blocked means. I'll add a new status for this.