main/postmarketos-base-ui: configure flathub from file drop-in

Per the flatpak manpage, repos can be configured OOTB by dropping config under /etc/flatpak/remotes.d. Using the install script requires doing stuff over the internet, which may not always work at image build time. By using the drop-in file, we avoid having to connect to flathub.

The manpage only mentions /etc/flatpak, and nothing about /usr ...