support for asymmetric crypto keys and new signature format

support for asymmetric crypto keys and new signature format

Asymmetric keys were introduced in linux-3.7 to verify the signature on
signed kernel modules. The asymmetric keys infrastructure abstracts the
signature verification from the crypto details. Asymmetric crypto keys
support allows to import X509 public key certificate in a DER format
into the kernel keyring. Asymmetric keys require a new signature format.
'evmctl -x' or 'evmctl --x509' option can be used to utilize new
signature format.

Using of key filename after the file name for signing and verification commands
is a bit odd. This patch add '--key' parameter to specify non-default key file.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>