Signed-off-by: Michael Brown <mcb30@ipxe.org>

The Cryptographic Message Syntax (PKCS#7) provides a format for
encapsulating digital signatures of arbitrary binary blobs.  A
signature can be generated using

  openssl cms -sign -in <file to sign> -binary -noattr \
  	  -signer <signer>.crt -inkey <signer>.key -certfile <CA>.crt \
	  -outform DER -out <signature>

Signed-off-by: Michael Brown <mcb30@ipxe.org>