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 PR adds:

• Support for reading temperature on MAX2837, MAX2839 and MAX2831.
  • Switches the RSSI MUX to output the analog temperature, and samples with the LPC43xx ADC, giving ~0.3 degrees per LSB.
• 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: 24°C
There is 1 other device on the same USB bus.
You may have problems at high sample rates.

[martinling]

@miek I've revised this to use the same reading method on all boards - connect the analog temperature output to the LPC43xx ADC and read it that way. That gets us better resolution than was possible with the built in 5-bit ADC on the MAX2837 and MAX2839.

I'm getting responsive and generally similar readings on HackRF One r4 & r9, HackRF Pro, and rad1o, but the absolute numbers feel a bit high, so I think we still need to do some calibration.

[martinling]

I've calibrated this as best as I can with a temperature probe and a hot plate. Refinements welcome :)