[crypto] Add concept of additional data to cipher algorithms

[crypto] Add concept of additional data to cipher algorithms

Some ciphers (such as GCM) support the concept of additional
authenticated data, which does not appear in the ciphertext but may
affect the operation of the cipher.

Allow cipher_encrypt() and cipher_decrypt() to be called with a NULL
destination buffer in order to pass additional data.

Signed-off-by: Michael Brown <mcb30@ipxe.org>