Add support for reading transceiver temperature

[martinling]

All the MAX283x chips include a built-in temperature sensor, but we've never had support for reading it out in the standard firmware.

This is in draft until the values are correctly calibrated!

This PR adds:

• Functions for reading temperature on MAX2837, MAX2839 and MAX2831.
  • MAX2837 and MAX2839 have built-in 5-bit ADCs.
    • Currently just the raw ADC value is returned, this needs to be calibrated.
  • MAX2831 requires switching the RSSI MUX to output the analog temperature, and sampling with the LPC43xx ADC.
    • Conversion to degrees is based on datasheet values and may need adjustment.
• A new hackrf_read_temperature function to libhackrf.
• Transceiver temperature to hackrf_info:

$ hackrf_info 
hackrf_info version: git-6d545835
libhackrf version: git-6d545835 (0.10.0)
Found HackRF
Index: 0
Serial number: 0000000000000000977c64de390d7813
Board ID Number: 5 (HackRF Pro)
Firmware Version: git-8dbb3475 (API:1.13)
Part ID Number: 0xa0000a30 0x00664f62
Hardware Revision: r1.2
Hardware appears to have been manufactured by Great Scott Gadgets.
Hardware supported by installed firmware:
    HackRF One
    HackRF Pro
Transceiver temperature: 43°C
There is 1 other device on the same USB bus.
You may have problems at high sample rates.