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/configs/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index c227ef4..3cd40e9 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -25,3 +25,4 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_SYS_NS16550=y
 CONFIG_OF_LIBFDT=y
+CONFIG_SPL_NET_VCI_STRING="AM335x U-Boot SPL"