kicad-unstable-small: init to make kicad-unstable

the -small packages depend on all hydra buildable dependencies
the non-small ones depend on packages3d which exceeds hydra's limit

set platforms to all (kicad is cross-platform)
clarify package differences in the description

set maintainers on just the top level derivation

switch -unstable to not save debug symbols

indicate patch in version string

note broken dependencies