Convert CONFIG_SPL_NET_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/pengwyn_defconfig b/configs/pengwyn_defconfig
index 88c5a2b..02b44fc 100644
--- a/configs/pengwyn_defconfig
+++ b/configs/pengwyn_defconfig
@@ -14,6 +14,7 @@
 CONFIG_SPL_ETH_SUPPORT=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_NET_SUPPORT=y
 CONFIG_SPL_NET_VCI_STRING="AM335x U-Boot SPL"
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y