Avr fixes

The avr-raven platform is in a bad shape. Some projects, e.g. er-rest-example, do not even compile.

Another issue is that when used with the Cetic 6lbr border router, the computation of the RPL DAO lifecycle timer was incorrect due to the limited clock_time_t width of the avr-raven and the default, relatively large values, of the DIO default lifecycle and lifecycle unit used by the Cetic 6lbr (30 and 256 respectively).

Finally, with recent avr-gcc compiles, the avr-ravelcd arrays need to be declared const.