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/Makefile b/boot/Makefile
index f990e66..43baf2b 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -30,7 +30,7 @@
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_SCRIPT) += bootmeth_script.o
 ifdef CONFIG_$(SPL_TPL_)BOOTSTD_FULL
 obj-$(CONFIG_$(SPL_TPL_)CMD_BOOTEFI_BOOTMGR) += bootmeth_efi_mgr.o
-obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootflow_menu.o
+obj-$(CONFIG_$(SPL_TPL_)EXPO) += bootflow_menu.o
 endif
 
 obj-$(CONFIG_$(SPL_TPL_)OF_LIBFDT) += image-fdt.o
@@ -48,7 +48,7 @@
 obj-$(CONFIG_SPL_LOAD_FIT) += common_fit.o
 endif
 
-obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE) += expo.o scene.o scene_menu.o
+obj-$(CONFIG_$(SPL_TPL_)EXPO) += expo.o scene.o scene_menu.o
 
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE) += vbe.o vbe_request.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE_SIMPLE) += vbe_simple.o