boot: Introduce BOOTSTD_MENU to control bootflow menu build

The bootflow_menu.c code depends on e.g. scene_txt_set_font(),
which is only built when CONFIG_EXPO is enabled. Introduce new
Kconfig symbol BOOTSTD_MENU which depends on EXPO to prevent
triggering errors like these in case e.g. CONFIG_VIDEO=n :

"
boot/bootflow_menu.c:158:(.text+0x8851): undefined reference to `scene_txt_set_font'
"

Make the symbol depend on BOOTSTD_FULL as well to get rid of
the Makefile dependency workaround. Since BOOTSTD_FULL is not
available in SPL, do not define SPL variant of BOOTSTD_MENU.

Fix up bootflow test accordingly.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index fe2919f..b168b6b 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -469,6 +469,13 @@
 	  standard boot does not support all of the features of distro boot
 	  yet.
 
+config BOOTSTD_MENU
+	bool "Provide a menu of available bootflows for standard boot"
+	depends on BOOTSTD_FULL && EXPO
+	default y
+	help
+	  Provide a menu of available bootflows and related options.
+
 config BOOTSTD_PROG
 	bool "Use programmatic boot"
 	depends on !CMDLINE