Fix dtc submodule build error when using gcc -g or -O0

The build error:
/usr/bin/ld: /tmp/cc2eaHdM.ltrans0.ltrans.o: in function fdt_del_node':
<artificial>:(.text+0x279f0): undefined reference to fdt_node_end_offset_'
collect2: error: ld returned 1 exit status

Author: ChinYikMing

mk/system.mk

@@ -28,7 +28,7 @@ DEV_OBJS := $(patsubst $(DEV_SRC)/%.c, $(DEV_OUT)/%.o, $(wildcard $(DEV_SRC)/*.c
 deps := $(DEV_OBJS:%.o=%.o.d)
 
 OBJS_EXT += system.o
-OBJS_EXT += dtc/libfdt/fdt.o dtc/libfdt/fdt_ro.o dtc/libfdt/fdt_rw.o
+OBJS_EXT += dtc/libfdt/fdt.o dtc/libfdt/fdt_ro.o dtc/libfdt/fdt_rw.o dtc/libfdt/fdt_wip.o
 
 # system target execution by using default dependencies
 LINUX_IMAGE_DIR := linux-image