sunxi: make SoC variant choice mandatory

The user should always select an SoC variant to support. Not choosing
one doesn't make sense for a bootloader.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

board/sunxi/Kconfig

@@ -18,7 +18,6 @@ config SUNXI_GEN_SUN6I
 
 choice
 	prompt "Sunxi SoC Variant"
-	optional
 
 config MACH_SUN4I
 	bool "sun4i (Allwinner A10)"