ubuntu-image syntax for local gadget snap

[Dave Breed]

Hi,

I'm trying to build an Snappy image for my armhf using the
ubuntu-image tool. I've rolled my own kernel and gadget snaps, and
model assertion file. I understand that I can use the --extra-snaps
argument on the command line to specify a path to a local kernel snap,
but I'm not clear if the same can be done for the gadget snap? I've
tried using --extra-snaps and, alternatively, specifying a local
absolute path to the snap in the assertion file, but either way
ubuntu-image just returns an error saying the gadget snap can't be
found.

I'm probably just missing something really obvious here, but any help
with the correct method would appreciated.

Thanks,

Dave.