Hi @jean,
First of all welcome to Mender Hub.
Second, we recently publish a reference integration for the imx8mqevk board,
and there is also an demo manifest file in the i.MX BSP,
https://source.codeaurora.org/external/imx/imx-manifest/tree/README-mender?h=imx-linux-sumo
But to get to the bootloader configuration, for the imx8mqevk
board we did the following,
And I suspect it would be something similar on the imx8mmevk