(Luna) Add default wallpaper as symlink
Adds a default wallpaper from the artwork package to Luna. (Final step to #1111, #1113)
Tested in QEMU & on teclast-x80pro by starting Luna and ensuring the wallpaper is as expected.
Sorry this took so long - I'm back to school for a few months and rather short on free time. Haven't had a chance to look at much postmarketOS stuff recently.