net: Move CONFIG_SPL_NET_VCI_STRING into Kconfig

This patch also adds the SPL time VCI string into Kconfig.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/net/Kconfig b/net/Kconfig
index 414c549..c393269 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -45,4 +45,7 @@
 	default "U-Boot.arm" if ARM
 	default "U-Boot"
 
+config SPL_NET_VCI_STRING
+	string
+
 endif   # if NET