[U-Boot] dtb build issue on U-Boot

I notice that the dtb content in spl/u-boot-spl.bin and u-boot.bin not

able to update after I modify the dts file and make again, I have to

"make clean" first, and then the "make" can update the dtb content

to output binary, which lead to bad efficiency when try to modify dts.

I have try below command, the dtb file also not updated before "make clean".

dtc -I dtb -O dts spl/u-boot-spl.dtb

Is there any one can help fix this issue?

Thanks,

- Kever