There is no img file in build output directory

Hello. I'm newbie in OpenWrt,

I'm trying to build OpenWrt for single board computer OK1046A-C2, which uses LS1046A.

I set Target profile to LS1046ARDB. And other configuration is set properly.
I could build OpenWrt with no error. But There is no objective img file in bin/targets/layerscape/armv8_64b.

If I set Target Profile to Multiple devices, I could get some img file like ext4-sdcard.img which I do not want. But there is no ext4 image without sdcard boot.

I read this section. Is that the cause that the image is too big?
If so, how can I avoid the problem?

Do not preinstall extra packages.
You can build them to install later.
Use extroot if necessary.