spl: Kconfig: Rename SPL_USBETH_SUPPORT to SPL_USB_ETHER to match with the U-boot CONFIG

Rename CONFIG_SPL_USBETH_SUPPORT to CONFIG_SPL_USB_ETHER.

This enables users to block text using CONFIG_IS_ENABLED() instead
of resorting to #if ladders with SPL and non-SPL cases.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
diff --git a/board/birdland/bav335x/board.c b/board/birdland/bav335x/board.c
index f284568..d4bb1e7 100644
--- a/board/birdland/bav335x/board.c
+++ b/board/birdland/bav335x/board.c
@@ -360,7 +360,7 @@
  * Build in only these cases to avoid warnings about unused variables
  * when we build an SPL that has neither option but full U-Boot will.
  */
-#if ((defined(CONFIG_SPL_ETH_SUPPORT) || defined(CONFIG_SPL_USBETH_SUPPORT)) &&\
+#if ((defined(CONFIG_SPL_ETH_SUPPORT) || defined(CONFIG_SPL_USB_ETHER)) &&\
 		defined(CONFIG_SPL_BUILD)) || \
 	((defined(CONFIG_DRIVER_TI_CPSW) || \
 	  defined(CONFIG_USB_ETHER) && defined(CONFIG_USB_MUSB_GADGET)) && \