postmarketos-mkinitfs: don't depend on osk-sdl, add to initfs conditionally (MR 2242)

osk-sdl is explicitly installed by pmbootstrap:
https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/2066

As such, mkinitfs should conditionally add osk-sdl and its dependencies
to the initfs, based on whether it is installed or not.

This is kind of a 'quick and dirty' approach, the test is simple enough
(-x /usr/bin/osk-sdl) that it didn't seem worth adding a separate
function just for that.