mkosi: let's remove if checks from build_image() a bit
mkosi: let's remove if checks from build_image() a bit

Let's move most remaining if conditions into the functions we are about
to call. This way the flow of control of build_image() is easier to
follow as the exceptions are hidden and the common flow of control
emphasised.

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