I would like to create AMI for an instance for 1 volume not for all volumes using boto. I've a script to automate AMI creation for couple of instances. However one of the instance consisting of huge volume for backups(No worries about that data). We would like to take snapshot of root volume in AMI creation not for the other volumes. is there any way to do this?
If the backup volume is a separate EBS/SSD volume, you may have chance to create a small snapshot from your root volume. You just need to unmount it from (as in Linux) OS level.

Load the instance OS
Unmount the huge volume from the OS level
shutdown  OS
Take a snapshot
Reload the instance
Mount the volume back

However, this will not work if your backup volume is also part of the your root instance volume. 
Important note : DO NOT run any AWS detach-volume command. An OS unmount is not AWS detach-volume.
How to Create AMI for 1 volume using boto?