[Merge] livecd-rootfs:sil2100/core18-pi3-arm64 into livecd-rootfs:ubuntu/master

[Łukasz Zemczak]

Łukasz Zemczak has proposed merging livecd-rootfs:sil2100/core18-pi3-arm64 into livecd-rootfs:ubuntu/master.

Commit message:
Support building pi3-arm64 core18 images. At the same time, make sure we still use the right gadget tree repository for the classic arm64 pi3 image.

Requested reviews:
  Ubuntu Core Development Team (ubuntu-core-dev)

For more details, see:
https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/362825

Support building pi3-arm64 core18 images. At the same time, make sure we still use the right gadget tree repository for the classic arm64 pi3 image.

Basically without this change we can't build pi3 arm64 core18 images. The additional conditional is needed because basically not always the model name will fit the gadget tree git repo name. For instance in this case - the model name is pi3-arm64 but it's built from a common pi repository, so we can't suddenly start looking for the gadget tree under the pi3-arm64 name as such repo does not exist.
This conditional can be useful for other platforms once those pop up, which is why I used a case statement there.

I was also considering simply changing the first $ARCH+${SUBARCH:-} switch case to something like $PROJECT+$ARCH+${SUBARCH:-} instead, but that seemed a bit less clean to me in the end.
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of livecd-rootfs:sil2100/core18-pi3-arm64 into livecd-rootfs:ubuntu/master.
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