Add support for OpenSSL ENGINE for keystore

keystore now has an OpenSSL ENGINE frontend that can be used to ask for
private keys or public keys to be loaded and also signing operations to
take place. Use that instead of the crazy byte-array marshalling of the
private key material that is used for the "privsep" stuff.

Change-Id: I6171ca1fb0e77e338c19f04d8c34ad7744984b63
3 files changed