am335x_evm: Clarify when we build board_eth_init

If we build this function in cases where we would be discarding it
anyhow we still end up with maybe unused warnings.  Rather than litter
the function with __maybe_unused, just spell out when to build it.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/board/ti/am335x/board.c b/board/ti/am335x/board.c
index 7609a18..554398f 100644
--- a/board/ti/am335x/board.c
+++ b/board/ti/am335x/board.c
@@ -573,8 +573,22 @@
 };
 #endif
 
-#if defined(CONFIG_DRIVER_TI_CPSW) || \
-	(defined(CONFIG_USB_ETHER) && defined(CONFIG_MUSB_GADGET))
+/*
+ * This function will:
+ * Read the eFuse for MAC addresses, and set ethaddr/eth1addr/usbnet_devaddr
+ * in the environment
+ * Perform fixups to the PHY present on certain boards.  We only need this
+ * function in:
+ * - SPL with either CPSW or USB ethernet support
+ * - Full U-Boot, with either CPSW or USB ethernet
+ * 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)) \
+		&& defined(CONFIG_SPL_BUILD)) || \
+	((defined(CONFIG_DRIVER_TI_CPSW) || \
+	  defined(CONFIG_USB_ETHER) && defined(CONFIG_MUSB_GADGET)) && \
+	 !defined(CONFIG_SPL_BUILD))
 int board_eth_init(bd_t *bis)
 {
 	int rv, n = 0;