main/postmarketos-base: Use deviceinfo_getty to configure a getty

Configure a getty by setting the port and baud rate in a new deviceinfo variable, deviceinfo_getty. The format is "<port>;<baud rate>". For example, "ttyS0;115200".

A post-installation trigger in postmarketos-base checks /etc/deviceinfo, and modifies /etc/inittab if the device should run a getty.

Fixes #268 (closed)