mkimage: fit: only process one cipher node

Previously mkimage would process any node matching the regex cipher.*
and apply the ciphers to the image data in the order they appeared in
the FDT. This meant that data could be inadvertently ciphered multiple
times.

Switch to processing a single cipher node which exactly matches
FIT_CIPHER_NODENAME.

Signed-off-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>