Reduce travis overhead for testing 6502 ports.

- build cc65 libraries only for target supported by Contiki
- instead of building all examples for all 6502 targets build
  - all examples for one target (c64)
  - the most demanding client example (webbrowser) for all 6502 targets
  - the most demanding server examples (webserver) for all 6502 targets
Merge pull request #445 from oliverschmidt/master

Reduce travis overhead for testing 6502 ports.