appended signatures: Introducing key management environment variable

appended signatures: Introducing key management environment variable

Introducing the appended signature key management environment variable. It is
automatically set to either "static" or "dynamic" based on the Platform KeyStore.

"static": Enforce static key management signature verification. This is the
          default. When the GRUB is locked down, user cannot change the value
          by setting the appendedsig_key_mgmt variable back to "dynamic".

"dynamic": Enforce dynamic key management signature verification. When the GRUB
           is locked down, user cannot change the value by setting the
           appendedsig_key_mgmt variable back to "static".

Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>