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/pcm051_rev3_defconfig b/configs/pcm051_rev3_defconfig
index f6ff370..b8b391d 100644
--- a/configs/pcm051_rev3_defconfig
+++ b/configs/pcm051_rev3_defconfig
@@ -13,6 +13,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_ETH_SUPPORT=y
+CONFIG_SPL_NET_VCI_STRING="pcm051 U-Boot SPL"
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set