There are multiple other openssl engines used by HSMs that can be used to
sign FIT images instead of forcing users to use pkcs11 type of service.

Relax engine selection so that other openssl engines can be specified and
use generic key id definition formula.

Signed-off-by: Vesa Jääskeläinen <vesa.jaaskelainen@vaisala.com>
Cc: Tom Rini <trini@konsulko.com>