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/google/Kconfig b/board/google/Kconfig
index 302f68e..e9559c9 100644
--- a/board/google/Kconfig
+++ b/board/google/Kconfig
@@ -8,6 +8,7 @@
 
 choice
 	prompt "Mainboard model"
+	optional
 
 config TARGET_CHROMEBOOK_LINK
 	bool "Chromebook link"