change ci-img target to build desktop, server and demo
New targets desktop, server and demo, replacing previous ci-img outputs. Names must match @.toml filesystem config. Makes harddrive.img and livedisk.iso for each, copying them to build/img/redox_
@_$TAG_harddrive.img etc. To create e.g. redox_demo_harddrive.img with no tag, set IMG_TAG= and IMG_SEPARATOR= .