libxbee cleanup and update

This is not ready for merge but open for discussion

- The goal is to update libxbee and to compile it together with qgroundcontrol instead of providing binary files
- Because we do the linking statically no 'make install' of libxbee should be necessary
- With the update of libxbee some interface code in qgc was changed which compiles but is untested
- This builds on Linux but was not yet tested on other operating systems