Add enable_uart option to snapd for Raspberry Pi

[Jenny Murphy]

By default the serial port is disabled on Raspberry Pi3. See description here:

https://www.raspberrypi.org/documentation/configuration/uart.md

"The Linux console can be re-enabled by adding enable_uart=1 to config.txt. This also fixes the core_freq to 250Mhz (unless force_turbo is set, when it will fixed to 400Mhz), which means that the UART baud rate stays consistent."

The config.txt file is writable so it can be manually edited on the installed image as a workaround.

To implement it properly the option should be added to:

snapcore/snapd/blob/master/overlord/configstate/configcore/picfg.go

then it can be set via

snap set core pi-config.enable_uart=1
or via a respective defaults: entry in a custom gadget in the gadget.yaml

[Zygmunt Krynicki (zyga)]

This bug is split between the raspberry pi 3 gadget snap and the snapd snap which implements some of the raspberry pi specific config options. I'll ask my colleagues to look at this.