bootstd: Allow enabling BOOTSTD_FULL without needing EXPO

It is sometimes useful to have one without the other, e.g. on a device
without a display, since at present the expo feature requires CONFIG_VIDEO
to be enabled.

Update the Makefile and bootflow command to support this, as well as the
EXPO dependency.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index fdcfbae..98e194f 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -561,6 +561,7 @@
 
 config EXPO
 	bool "Support for expos - groups of scenes displaying a UI"
+	depends on VIDEO
 	default y if BOOTMETH_VBE
 	help
 	  An expo is a way of presenting and collecting information from the