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/dbau1x00/Kconfig b/board/dbau1x00/Kconfig
index 1286e45..b813adb 100644
--- a/board/dbau1x00/Kconfig
+++ b/board/dbau1x00/Kconfig
@@ -13,6 +13,7 @@
 
 choice
 	prompt "Select au1x00 SoC type"
+	optional
 
 config DBAU1100
 	bool "Select AU1100"