net: Fix NET_RANDOM_ETHADDR dependencies

NET_RANDOM_ETHADDR depends on lib/rand.c. This patch adds dependency to
Kconfig to ensure that library is also compiled.

Remove the definitions from Blackfin boards' include/configs.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>