Weston config should be loaded on start

Currently the config of weston is loaded post-install. This is done in postmarketos-ui-weston.post-install.

Whenever changing one of the weston config values in the deviceinfo files, you should explicitly reinstall postmarketos-ui-weston (this is not done automatically). However, this does not make much sense because the deviceinfo file is in a device-* package, so you suspect only having to rebuild the device-* package.

Therefore I propose to load the weston config when starting weston, in the startup_weston.sh script.