MODSIGN: Add -s <signature> option to sign-file

This option allows to append an externally computed singature to the
module. This is needed in setups, where the private key is not directly
available, but a service exists that returns signatures for given files.

Signed-off-by: Michal Marek <mmarek@suse.cz>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>