Add initial Doxygen file

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
Document bt-daemon.h

bt-daemon.h is now complete in its documentation, which can be
generated using doxygen

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
Document bt-daemon-private.h

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
Document lbuxton.c

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
Add initial src/cli/main.c documentation

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
Add initial buxtond documentation

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
Add Internal API link to mainpage

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>