image: Allow loading a FIT image for a particular phase

image: Allow loading a FIT image for a particular phase

Add support for filtering out FIT images by phase. Rather than adding yet
another argument to this already overloaded function, use a composite
value, where the phase is only added in if needed.

The FIT config is still selected (and verified) as normal, but the images
are selected based on the phase.

Tests for this come in a little later, as part of the updated VPL test.

Signed-off-by: Simon Glass <sjg@chromium.org>