targets: move target options to their own sub-menu

Currently, all target options lie in the top-level menu.
This looks a bit clumsy, since all other options are neatly
folded into their own sematic sub-menus each.

Move the target option into their own sub-menu too.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

arch/Config.in

@@ -1,3 +1,5 @@
+menu "Target options"
+
 config BR2_ARCH_IS_64
 	bool
 
@@ -335,3 +337,5 @@ endif
 if BR2_xtensa
 source "arch/Config.in.xtensa"
 endif
+
+endmenu # Target options