fiptool: Add support for printing the sha256 digest with info command

This feature allows one to quickly verify that the expected
image is contained in the FIP without extracting the image and
running sha256sum(1) on it.

The sha256 digest is only shown when the verbose flag is used.

This change requires libssl-dev to be installed in order to build
Trusted Firmware. Previously, libssl-dev was optionally needed only
to support Trusted Board Boot configurations.

Fixes ARM-Software/tf-issues#124

Change-Id: Ifb1408d17f483d482bb270a589ee74add25ec5a6