I2C port configurable

[hreuver]

With "cat /proc/cpuinfo" or "i2cdetect -l" most information can be obtained. If you need more information, please ask.

Below you find:
- RPI4 (for reference)
- Odroid N2 (my target)
- RockPro64 (I probably won't use it with openplotter-i2c, but it was here).
I shortened the information a little since most data is simply the same information with a different core number.

I think more generic code for openplotter-i2c, openplotter-can and openplotter-pypilot is a good thing of itself. A configurable I2C port would be very helpfull even without explicit support for my system.

<off-topic>
My intended use would be the compass-only mode, providing pressure and heading from a MPU9250 + BMP280 to SignalK.

Currently RTIMULibDemoGL works without a problem with a MPU9250 (with SignalK in the background, being fed from from other sensors via openplotter-i2c and canboat). However when I try openplotter-pypilot the i2c-bus gets blocked.

I suspect I missed some i2c command while trying to get openplotter-pypilot to work. I will report it if I find the cause (even without support).
</off-topic>

==========
Raspberry Pi 4:
$ cat /proc/cpuinfo
processor : 0
model name : ARMv7 Processor rev 3 (v7l)
BogoMIPS : 270.00
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm crc32
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xd08
CPU revision : 3
..
Hardware : BCM2835
Revision : b03111
Serial : 100000002ad088c4
Model : Raspberry Pi 4 Model B Rev 1.1
$ i2cdetect -l
i2c-1 i2c bcm2835 I2C adapter I2C adapter

==========
The Odroid:
$ cat /proc/cpuinfo
processor : 0
BogoMIPS : 48.00
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 4
..
processor : 5
BogoMIPS : 48.00
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd09
CPU revision : 2

CPU info : 290a4000010e1900000c303254524d50
Serial : 4682a6ca-f9ce-4159-b157-001e06423e0b
Hardware : Hardkernel ODROID-N2
Revision : 0400

(AmLogic S922X: 2x A53 like the RPI3 + 4x A73)
$ i2cdetect -l
i2c-3 i2c Meson I2C adapter I2C adapter
i2c-2 i2c Meson I2C adapter I2C adapter

Mostly the i2c-2 is used.
The second i2c-bus is used for the RTC (but is is also accessible through the 40pin header).

==========
The RockPro64:

$ cat /proc/cpuinfo
..
processor : 3
BogoMIPS : 48.00
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 4

processor : 4
BogoMIPS : 48.00
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd08
CPU revision : 2

processor : 5
BogoMIPS : 48.00
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd08
CPU revision : 2

Serial : 0000000000000000

(RK3399: 4x A53 like the RPI3 and 2x A72 like the RPI4)

# i2cdetect -l
i2c-0 i2c rk3x-i2c I2C adapter
i2c-1 i2c rk3x-i2c I2C adapter
i2c-4 i2c rk3x-i2c I2C adapter
i2c-8 i2c rk3x-i2c I2C adapter
i2c-9 i2c DesignWare HDMI I2C adapter
i2c-10 i2c DP-AUX I2C adapter

I assume i2c-8 is available through the Pi2 40pins connector.