net: Remove legacy Contiki based uIP stack

This commit removes the legacy Contiki based uIP stack.
The new native IP stack must be used after this commit.

The commit also removes following things:
- legacy cc2520 driver
- legacy ethernet drivers
- legacy IP stack samples

and changes these things:
- disabled tests that only work for legacy IP stack
- select new IP stack by default
- enable random number generator by default as it is needed
  by the new IP stack

Change-Id: I1229f9960a4c6654e9ccc6dac14a7efb9394e45d
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>