Introduce Verifying Program Loader (VPL)

Add support for VPL, a new phase of U-Boot. This runs after TPL. It is
responsible for selecting which SPL binary to run, based on a
verified-boot process.

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

scripts/Kbuild.include

@@ -322,11 +322,15 @@ endif
 
 ifdef CONFIG_SPL_BUILD
 SPL_ := SPL_
+ifeq ($(CONFIG_VPL_BUILD),y)
+SPL_TPL_ := VPL_
+else
 ifeq ($(CONFIG_TPL_BUILD),y)
 SPL_TPL_ := TPL_
 else
 SPL_TPL_ := SPL_
 endif
+endif
 else
 SPL_ :=
 SPL_TPL_ :=