mkosi: use debootstrap's --merged-usr switch only if supported
mkosi: use debootstrap's --merged-usr switch only if supported

This allows us to run mkosi on Semaphore, which only provides us with
Ubuntu 16.04 as newest release, where this swich isn't known yet.

UbiCastTeam/mkosi,UbiCastTeam/mkosi,systemd/mkosi,systemd/mkosi