Use keystore ENGINE for private key operations

Use keystore ENGINE for private key operations

The new keystore ENGINE is usable to perform private key operations when
we can't get the actual private key data. This is the case when hardware
crypto is enabled: the private key never leaves the hardware.

Subsequently, we need to be able to talk to OpenSSL ENGINEs that aren't
PKCS#11 or OpenSC. This just changes a few #define variables to allow us
to talk to our keystore engine without having one of those enabled and
without using a PIN.

Change-Id: Iabab5077c3d167a1e13bc8ef8745dc59ad4d62f7