Give geniv its own Kconfig option so that its dependencies are
distinct from that of the AEAD API code.  This also allows it
to be disabled if no IV generators (seqiv/echainiv) are enabled.

Remove the obsolete select on RNG2 by SKCIPHER2 as skcipher IV
generators disappeared long ago.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>