Update Makefile and Documentation for release

[lydon42]

The Makefile needs a bigger cleanup and also some of the documentation files need an update. This bundles a bunch of issues.

Tasks:

• [ ] CX65 variables should only refer to the binary, move options into CX65OPTS variables
• [ ] pass CX65 variables to submodules (freezemenu, fdisk), so that they don't need to build cc65, too
• [x] update target dependecies
  • [x] always submodule Makefile!
  • [x] check all targets if they really build a new version if some part is changed
• [ ] cleanup clean target and make clean-vivado subtarget
  • [ ] vivado cache rm is missing all the new targets!
• [x] cleanup requirements in docs/build.md
  • [x] remove or update versions (python 2.X not cool!)
  • [x] ImageMagick
  • [x] libtinfo
  • [x] exomizer

[lydon42]

Force submodule Makefile execution: 6b57441e1fa8e56ac7506bc55969b9744848ff90

[dansanderson]

I rewrote docs/build.md in August 2023, so those checkboxes can be ticked. https://github.com/MEGA65/mega65-core/blob/development/docs/build.md

[lydon42]

And I did check all the dependencies for 0.96, so those can also be checked.