Some Arcadyan devices (e.g. MTS WG430223) keep their config in encrypted
mtd. This adds mtd_get_mac_encrypted_arcadyan() function to get the MAC
address from the encrypted partition. Function uses uencrypt utility for
decryption (and openssl if the uencrypt wasn't found).
Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>