Kconfig: Add a 'Boot options' menu

There are quite a few boot-related menu options at the top level. Create a
new menu to hold these and move 'Boot images' into it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Kconfig b/Kconfig
index 1a132f9..34f17eb 100644
--- a/Kconfig
+++ b/Kconfig
@@ -432,6 +432,8 @@
 
 endmenu		# General setup
 
+menu "Boot options"
+
 menu "Boot images"
 
 config ANDROID_BOOT_IMAGE
@@ -769,6 +771,8 @@
 
 endmenu		# Boot images
 
+endmenu		# Booting
+
 source "api/Kconfig"
 
 source "common/Kconfig"