Improve Pi detection

[a-martynovich]

/proc/cpuinfo in Ubuntu Core and Server does not contain "Hardware" and "Serial" fields. Instead some other files should be queried.

To get device model we can read /proc/device-tree/model. It contains data like Raspberry Pi 3 Model B Plus Rev 1.3\0 (notice the null byte at the end). Serial number is at /proc/device-tree/serial-number. Also /proc/device-tree/compatible contains info in such format: raspberrypi,3-model-b-plus brcm,bcm2837.

[a-martynovich]

This also works the same way on Raspbian.