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>
diff --git a/doc/uImage.FIT/howto.txt b/doc/uImage.FIT/howto.txt
index 019dda2..6dbd17d 100644
--- a/doc/uImage.FIT/howto.txt
+++ b/doc/uImage.FIT/howto.txt
@@ -70,6 +70,12 @@
 loaded. The information about loadables locations is passed via the DT node with
 fit-images name.
 
+Finally, if there are multiple xPL phases (e.g. SPL, VPL), images can be marked
+as intended for a particular phase using the 'phase' property. For example, if
+fit_image_load() is called with image_ph(IH_PHASE_SPL, IH_TYPE_FIRMWARE), then
+only the image listed into the "firmware" property where phase is set to "spl"
+will be loaded.
+
 Loadables Example
 -----------------
 Consider the following case for an ARM64 platform where U-Boot runs in EL2