Right now, in python/vyos/version.py we have a custom check for boot type that uses the SquashFS file name (filesystem.squshafs or a version file like 1.3.7.squashfs.

But we already have a utility function for boot type checking: https://github.com/vyos/vyos-1x/blob/current/python/vyos/system/image.py#L251 and we could use it there.