thanks for the useful link and suggestion, JPMontero_Intel - looking at some other threads, I hadn't realized systemd could be involved at boot time (but all I want to do is pass some driver params, sorry if I was unclear)
alitt - thank you! I had this terrible suspicion it was going to be something like that - modem connection at boot time!
was hoping to be able to make adjustments from runtime, but ran into some issues
- missing fw_env.config (I'm on emutex/ubilinux) - no big problem: I crabbed an example from the yocto rootfs img file.
- I think this debian version of u-boot-tools may be outdated, it only knows how to deal with MTD not MMC
so it looks as if I either can try to patch/compile later u-boot-tools that understand about eMMC;
or simply deal with the bother and make changes over USB modem
anyways, thanks to you both!