Convert CONFIG_SPL_ETH_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 804c821..681672e 100644
--- a/configs/pengwyn_defconfig
+++ b/configs/pengwyn_defconfig
@@ -1,8 +1,9 @@
 CONFIG_ARM=y
 CONFIG_TARGET_PENGWYN=y
 CONFIG_SPL_ENV_SUPPORT=y
-CONFIG_SPL=y
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
+CONFIG_SPL_ETH_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set