Draft: main/postmarketos-mkinitfs: add support for keyboard detect script

This uses a script, /usr/bin/phys-keyboard-present, that is added to the initfs for determining whether or not to run osk-sdl with -x to hide the on-screen keyboard.

phys-keyboard-present is an (optional) script provided by device packages, since the implementation for detecting whether or not a keyboard is present will likely differ across different devices. The script exit with a return code of 0 if a physical keyboard is present, or non-zero if one is not present.

This patch requires osk-sdl >=0.65