Draft: add config option for setting display rotation

This will allow distros and/or users to force a certain display rotation on devices where it's not correctly detected/used by buffyboard directly.

The idea is that pmOS (or other distros) can install (from a device package) a .conf file to force rotation to be a certain way when we are confident that it'll only really ever be in one direction for that device.

This is basically a janky workaround for #14. It needs a bit more work to make it functional, so maybe it's just easier to fix the original problem @ #14?