am335x_evm: Add am335x_evm_usbspl build target

We add USB (RNDIS gadget) SPL support as a separate target.  We need to
pull out YMODEM support in order to be a small enough target binary.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/board/ti/am335x/board.c b/board/ti/am335x/board.c
index 441758f..f4b972b 100644
--- a/board/ti/am335x/board.c
+++ b/board/ti/am335x/board.c
@@ -73,7 +73,7 @@
 	return !strncmp(header.config, "SKU#02", 6);
 }
 
-static int board_is_gp_evm(void)
+static int __maybe_unused board_is_gp_evm(void)
 {
 	return !strncmp("A33515BB", header.name, 8);
 }