Having integrated mender on the NXP i.MX8MPlus myself, maybe I can help here.
Though admittedly I am still on Yocto kirkstone, so details may vary here.
As @TheYoctoJester pointed out, you definitely have to figure out how to configure mender so that the .sdimg can boot.
Do you boot from an SD card or from eMMC memory?
On my setup, I flash the bootloader (imx-boot) and the rootfs (.sdimg) to eMMC with NXP’s uuu tool. This is my mender configuration:
MENDER_STORAGE_DEVICE = "/dev/mmcblk2"
MENDER_STORAGE_TOTAL_SIZE_MB = "7000"
MENDER_UBOOT_STORAGE_DEVICE = "2"
MENDER_UBOOT_ENV_STORAGE_DEVICE_OFFSET = "4194304"
MENDER_PARTITION_ALIGNMENT = "4096"
MENDER_RESERVED_SPACE_BOOTLOADER_DATA = "32768"
MENDER_BOOT_PART_SIZE_MB = "64"
MENDER_DATA_PART_SIZE_MB = "2000"
MENDER_DATA_PART_FSTAB_OPTS:append = ",sync"
and the output of fdisk:
sh:~$ sudo fdisk -l
Disk /dev/mmcblk2: 7.28 GiB, 7820083200 bytes, 15273600 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x767191aa
Device Boot Start End Sectors Size Id Type
/dev/mmcblk2p1 * 8256 139327 131072 64M c W95 FAT32 (LBA)
/dev/mmcblk2p2 139328 5193751 5054424 2.4G 83 Linux
/dev/mmcblk2p3 5193752 10248175 5054424 2.4G 83 Linux
/dev/mmcblk2p4 10248176 15273599 5025424 2.4G 83 Linux
Disk /dev/mmcblk2boot0: 4 MiB, 4194304 bytes, 8192 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk /dev/mmcblk2boot1: 4 MiB, 4194304 bytes, 8192 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk /dev/mmcblk1: 1.94 GiB, 2080374784 bytes, 4063232 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x74b199ff
Device Boot Start End Sectors Size Id Type
/dev/mmcblk1p1 239 4063231 4062993 1.9G e W95 FAT16 (LBA)
I had problems too while integrating (concerning the u-boot environment and uuu), they have already been described and solved here on mender hub: SPL not booting into main u-boot when loading using NXPs UUU tool and mender-uboot is enabled - #6 by Ossian
If your setup is similar to mine, feel free to ask for more information. If you use the SD card though, I will probably not be of much help.