rockchip: Make it possible to define per board boot_targets

Some board may want to have a different boot priority(a laptop
may want to usb has the highest boot priority for third-part
os installation). So let the board can define it's own
boot_targets.

Signed-off-by: Andy Yan <andyshrk@163.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/include/configs/rockchip-common.h b/include/configs/rockchip-common.h
index 9b8ab3c..d5550a4 100644
--- a/include/configs/rockchip-common.h
+++ b/include/configs/rockchip-common.h
@@ -13,7 +13,9 @@
 
 #ifndef CONFIG_XPL_BUILD
 
+#ifndef BOOT_TARGETS
 #define BOOT_TARGETS	"mmc1 mmc0 nvme scsi usb pxe dhcp spi"
+#endif
 
 #ifdef CONFIG_ARM64
 #define ROOT_UUID "B921B045-1DF0-41C3-AF44-4C6F280D3FAE;\0"