arch: Make board selection choices optional

By making the board selections optional, every defconfig will include
the board selection when running savedefconfig so if a new board is
added to the top of the list of choices the former top's defconfig will
still be correct.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Cc: Tom Rini <trini@konsulko.com>
diff --git a/board/sunxi/Kconfig b/board/sunxi/Kconfig
index a60d028..c60c81d 100644
--- a/board/sunxi/Kconfig
+++ b/board/sunxi/Kconfig
@@ -18,6 +18,7 @@
 
 choice
 	prompt "Sunxi SoC Variant"
+	optional
 
 config MACH_SUN4I
 	bool "sun4i (Allwinner A10)"