main/unl0kr: upgrade to 3.1.0

Changes since the last packaged version are listed in https://gitlab.com/postmarketOS/buffybox/-/releases/3.0.0 and https://gitlab.com/postmarketOS/buffybox/-/releases/3.1.0.

3.0.0:

• misc: Share the same lvgl submodule (and version) between buffyboard and unl0kr
• misc: Update lvgl to git master (2023-03-21) and eliminate lv_drivers

3.1.0:

• feat(buffyboard): Handle input device connection/disconnection at runtime; adds new dependency libudev
• feat(buffyboard): Allow choosing theme via config and add all themes from unl0kr
• feat(buffyboard): Add fbdev force-refresh quirk via config
• feat(buffyboard): Allow disabling input devices via config
• feat(buffyboard): Add CLI flags for overriding geometry & DPI
• feat(buffyboard): Add CLI flag for verbose logging
• fix(unl0kr): Shutdown message box doesn't close on decline
• fix(unl0kr): Shutdown message box buttons and label are unstyled
• fix(unl0kr): Build fails when DRM is disabled
• misc: Update lvgl to git master (2023-04-10)

Mouse input doesn't work in qemu but I think this is a known issue.