Provide system for generating SSID and passphrase

[m-roberts]

SSID from RPi MAC address would produce the same hostname between OSes.
Passphrase being the same would likely also be useful - but it should at least be secure.

Potentially in the future, we may want to look into being able to configure this from before the OS even boots up. Recovery partition might be able to offer something useful in this area.

[m-roberts]

Currently the SSID is the output of hostname.
We want passphrase to be deterministic but difficult to determine if off-device. This means that MAC address is not well-suited for this purpose.

Here is how we can use the Raspberry Pi's serial to generate the passphrase:

serial=$(grep "Serial" /proc/cpuinfo | awk '{print $3}')
cropped_serial=$(echo "${serial: -8}" | tr '[:lower:]' '[:upper:]')

[m-roberts]

Given that it is likely that a Pi will not be on a network at the moment that it enters into AP mode, this means that it is possible for multiple pi-top devices in a localised environment to share the same SSID.

Rather than address this at the SSID level, it would be better to ensure that each machine has a unique hostname ID.