[Task]: Figure out if the board variable `FULL_DESKTOP=` is still in use

### Task description

Many boards use the variable `FULL_DESKTOP=` in their board config. I can't seem to find a reference in the code though. This is from the readme:

https://github.com/armbian/build/blob/c84703d3b440ff3d93febdddf65c7014b9329f28/config/boards/README.md?plain=1#L78-L81

On the contrary, the `HAS_VIDEO_OUTPUT` variable seems more important and is actually referenced in the code. Maybe we should replace all occurrencs of `FULL_DESKTOP` with `HAS_VIDEO_OUTPUT`?

https://github.com/armbian/build/blob/c84703d3b440ff3d93febdddf65c7014b9329f28/config/boards/README.md?plain=1#L66-L69

Start with the board configuration README section linked in the issue and search the repository for FULL_DESKTOP and HAS_VIDEO_OUTPUT, including board configs and build scripts. Compare their actual references and document whether FULL_DESKTOP is still used; if replacement is justified, update the affected occurrences and README consistently, then verify no stale references remain.

Written by the indexing model from the issue text.