Add 'clean' and 'gui' option to configure
Add 'clean' and 'gui' option to configure

- Add two new options, 'clean' and 'gui', to the configure script.
  'clean' removes CMakeCache.txt and CMakeFiles/ from the working
  directory before invoking cmake.
  'gui' launches cmake-gui after bootstrapping.

falbrechtskirchinger/avr32-cmake-template,falbrechtskirchinger/avr32-cmake-template