vbe: Support loading SPL images

vbe: Support loading SPL images

VBE needs to load different images from a FIT depending on the xPL phase
in use. The IH_PHASE value is used to select the image to load.

Add the required logic to handle this. For compatibility with the
SPL-loader driver, fill out a struct spl_image_info with the details
needed to boot the next phase.

This is good enough for VBE-simple but ABrec will need the full set of
bootstd features. So add a USE_BOOTMETH define to control this.

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