cpu/arm/k60: Implement random.h

Deleted random.c placeholder stub and replaced with a straightforward
implementation of a linear-congruential generator (LCG).
This is a very weak random number generator, but it is probably enough
for our uses (generating random timeouts in network protocols etc.)

The constants used (multiplier and increment) were taken from [1].

[1]: Entacher, Karl. A collection of selected pseudorandom number
     generators with linear structures. ACPC-Austrian Center for
     Parallel Computation, 1997.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Initialize the random library.

Seeds the PRNG with the CPU UID LSB during startup.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>