rockchip: Add USB to the default boot targets

Now that most rockchip SoC based board have usb host support, enable
USB boot targets by default.

Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Fixed build errors when CONFIG_CMD_USB not defined:
Signed-off-by: Simon Glass <sjg@chromium.org>

include/configs/rockchip-common.h

@@ -12,11 +12,20 @@
 #include <config_distro_defaults.h>
 
 /* First try to boot from SD (index 0), then eMMC (index 1 */
+#ifdef CONFIG_CMD_USB
 #define BOOT_TARGET_DEVICES(func) \
 	func(MMC, mmc, 0) \
 	func(MMC, mmc, 1) \
+	func(USB, usb, 0) \
 	func(PXE, pxe, na) \
 	func(DHCP, dchp, na)
+#else
+#define BOOT_TARGET_DEVICES(func) \
+	func(MMC, mmc, 0) \
+	func(MMC, mmc, 1) \
+	func(PXE, pxe, na) \
+	func(DHCP, dchp, na)
+#endif
 
 #define CONFIG_RANDOM_UUID
 #define PARTS_DEFAULT \