mkfs.ubifs: Add authentication support

This adds support for authenticated UBIFS images. In authenticated
images all UBIFS nodes are hashed as described in the UBIFS
authentication whitepaper. Additionally the superblock node contains a
hash of the master node and itself is cryptographically signed in a node
following the superblock node. The signature is in PKCS #7 CMS format.

To generate an authenticated image these options are necessary:

--hash-algo=NAME     hash algorithm to use for signed images
                     (Valid options include sha1, sha256, sha512)
--auth-key=FILE      filename or PKCS #11 uri containing the authentication key
                     for signing
--auth-cert=FILE     Authentication certificate filename for signing. Unused
                     when certificate is provided via PKCS #11

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>