common: add support to fallback to plain SHA256

In case crypt-based hashing is enabled this will be the default mechanism
that is used. If a user wants to have support for both, the environment
variable `bootstopusesha256` can be set to `true` to allow plain SHA256
based hashing of the password.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>
Reviewed-by: Simon Glass <sjg@chromium.org>