Seed the RNG on boot

[mtusnio]

Right now any rand() calls are not really random between boots, similarly seeding with time won't work due to the fact that we do not have any persistent knowledge about current time making it predictable when done at boot. One way to do it is seed the RNG with ca8210_get_random results and rely on cascoda radio.

Acceptance criteria:

• rand() calls provide different, random results between boots

This would fix ConnectivityFoundry/AwaLWM2M#354