First of all, thank you very much for sharing. The display I use is waveshare ILI9486 4inch TFT shield compatibility for arduino. The problem of this shield is SPI config switch is set to ICSP as default. In fact, the switch must not be placed in the side of ICSP. I have wasted my time for not caring about this fact.

Another problem is that fbtft is not available on latest armbian. So, I had to use Debian jessie 3.4 from Armbian Archive Storage, because fbtft_device is gone in linux kernel 5.4 (fbtft wiki).

Finally, it worked well with this command.

$ sudo modprobe fbtft_device custom name=fb_ili9486 gpios=dc:3,reset:0,led:6 speed=16000000 busnum=1 rotate=90
$ con2fbmap 1 8