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_debug:

$ hackrf_debug --temperature
Transceiver temperature: 26°C

[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 :)

[martinling]

Moved the CLI temperature reporting from hackrf_info to hackrf_debug as requested.

[miek]

Looks good!

I did find the temperatures reported deviate a bit from when I test with a thermal camera pointed at the IC:

MAX2837 (r4-p2) reported	Measured
29	33
35	39
43	50
50	57

MAX2839 (r9) reported	Measured
28	33
32	37
39	45
43	50

MAX2831 (r1.1.1) reported	Measured
21	32
26	36
32	42
41	50