examples/l2fwd-crypto: improve random key generator
This patch improve generate_random_key() function by replacing rand()
function with reading from /dev/urandom.
CID 120136 : Calling risky function (DC.WEAK_CRYPTO)
dont_call: rand should not be used for security related applications, as
linear congruential algorithms are too easy to break
Coverity issue: 120136
Signed-off-by: Piotr Azarewicz <piotrx.t.azarewicz@intel.com> Acked-by: Declan Doherty <declan.doherty@intel.com>