when i build the kernel on ubuntu 20.04, compile error and prompt :
arch/arm/boot/mkimage : no such file or dir.

but i check the file exist in dir. what happend really confused me. i google so many answer, and can't resolve this issue. finally i read one page that android kernel build need 32bit libs support.

so i install those lib :
sudo apt-get install libc6-i386 lib32stdc++6 lib32gcc1 lib32ncurses5 lib32z1

then everythings are ok.