boot_signer: allow verification using an external certificate

Add a -certificate parameter, which allows one to specify an external
public key certificate to use for verifying boot image signatures:

  boot_signer -verify boot.img -certificate cert.x509.pem

This makes it possible to confirm that the boot image has been signed
with a specific key.

Change-Id: I41f1a05e1c8be3bfac2a86678d81beaae0e371bb
1 file changed