examples/ipsec-secgw: change CBC IV generation

examples/ipsec-secgw: change CBC IV generation

NIST SP800-38A recommends two methods to generate unpredictable IVs
(Initilisation Vector) for CBC mode:
1) Apply the forward function to a nonce (ie. counter)
2) Use a FIPS-approved random number generator

This patch implements the first recommended method by using the forward
function to generate the IV.

Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>