Convert CONFIG_SPL_NET_VCI_STRING to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_NET_VCI_STRING

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/am43xx_evm_ethboot_defconfig b/configs/am43xx_evm_ethboot_defconfig
index a59125c..4459e77 100644
--- a/configs/am43xx_evm_ethboot_defconfig
+++ b/configs/am43xx_evm_ethboot_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_ETH_SUPPORT=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_NET_VCI_STRING="AM43xx U-Boot SPL"
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set