Handle generating weston.ini in post-install script (#201)

This PR automatically creates the weston.ini in the
postmarketos-base post-install script based on options set in the
deviceinfo file for a particular device. This replaces #191.
If weston.ini exists, then no modifications are made.

(Optional) options supported in this PR for deviceinfo are:
deviceinfo_weston_core_modules - defaults to NONE if none specified
deviceinfo_weston_core_backend - defaults to 'fbdev-backend.so' if none specified
deviceinfo_weston_keymap_rules - keymap rule, found under /usr/share/X11/xkb/rules/
deviceinfo_weston_keymap_model - keymap model