Update ... feeling like I'm losing my mind now.
fw_env.config must be sensitive to spaces or tabs or something.
I messed around with it some more and it magically started working with my imx8mm-evk board with fw_env.config containing:
/dev/mmcblk1 0x400000 0x1000
I don't know why that didn't work before ... I tried all kinds of things to get this to work and really can't explain why it just started working. To test, I used fw_setenv to set a variable and rebooted and stopped in u-boot and did printenv and sure enough the variable I set is there.
Regards,
Brian