vbe: Support VBE simple

Add support for VBE simple, which permits firmware update of a single
image stored in MMC or another block device.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Makefile b/boot/Makefile
index c52c969..67e3352 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -48,3 +48,4 @@
 endif
 
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE) += vbe.o
+obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE_SIMPLE) += vbe_simple.o