Does openwrt support imx7?
No, it does not.
Do you have image for imx6ul or imx6solox mpu?
You can find images for supported IMX6 platforms here
https://downloads.openwrt.org/snapshots/targets/imx6/generic/
There are just imx6dl and imx6q.Can't I use imx6ul or imx6solox?
It's possible. I used imx6ul with openwrt for tests.