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.
Agreed.

Also the result of the computation should be compared to the maximum value of clock_time_t for the platform before casting it to clock_time_t and passing it to ctimer_set.

ctimer_set(&instance->dao_lifetime_timer, (clock_time_t)expiration_time,
handle_dao_timer, instance);

The right fix should be:
if (expiration_time > INFINITE_TIME) {
    PRINTF("RPL: DAO lifetime timer %lu exceeds INFINITE_TIME\n", (unsigned long)expiration_time);
    expiration_time = INFINITE_TIME;
}

Anyway INFINITE_TIME is only defined for a few platforms. Defining a CLOCK_MAX per every platform in contiki-conf.h would be a good idea too, e.g.:

include <limits.h>
...
typedef unsigned short clock_time_t;
#define CLOCK_MAX USHRT_MAX