MLK-18654-2 imx: Change USB boot device type

The SPL SDP is configured as BOOT_DEVICE_BOARD, so when booting from
USB, change its type to BOOT_DEVICE_BOARD, so we can use SDP.

Signed-off-by: Ye Li <ye.li@nxp.com>

arch/arm/mach-imx/spl.c

@@ -136,7 +136,7 @@ u32 spl_boot_device(void)
 	case QSPI_BOOT:
 		return BOOT_DEVICE_NOR;
 	case USB_BOOT:
-		return BOOT_DEVICE_USB;
+		return BOOT_DEVICE_BOARD;
 	default:
 		return BOOT_DEVICE_NONE;
 	}