Hio @Shubham_Pushpkar
sorry to join the party too late, and probably late to save you some time working around the problems.
Generally running system from emmc on arm devices requires different approach to the way images are built.
ubuntu-image does not support this, not in straight forward way anyway. Hopefully with latest little kernel boot loader support added to snapd (bootloader: little kernel support) thing might move faster.
In general for devices supporting fastboot flashing, you want to build image way it has “image file per partition”
To breach the gab till ubuntu-image learns this magic, I have created temp tool called dragonboard-builder
( snap install dragonboard-builder). Ignore name as it was initially intended to build emmc images for dragon board. It has same syntax as ubuntu-image only it gives you image in form you want. Plus few more tweaks to the image 
once more sorry for late comment
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