Convert CONFIG_SPL_USBETH_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.
Signed-off-by: NSimon Glass <sjg@chromium.org>

configs/am335x_evm_usbspl_defconfig

@@ -2,13 +2,14 @@ CONFIG_ARM=y
 CONFIG_TARGET_AM335X_EVM=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_FIT=y
-CONFIG_SYS_EXTRA_OPTIONS="NAND,SPL_USBETH_SUPPORT"
+CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_MUSB_NEW_SUPPORT=y
 CONFIG_SPL_NET_SUPPORT=y
+CONFIG_SPL_USBETH_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set