tools: mkeficapsule: support generating dynamic GUIDs

Add support for generating GUIDs that match those generated internally
by U-Boot for capsule update fw_images when using dynamic UUIDs.

Dynamic UUIDs in U-Boot work by taking a namespace UUID and hashing it
with the board compatible and fw_image name. This feature just provides
a way to determine the UUIDs for a particular board without having to
actually boot U-Boot on it.

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>