Merge lp://staging/~xnox/ubuntu-cdimage/focal-oem into lp://staging/ubuntu-cdimage

Tested locally that unittests pass, and that download completes:

===== Downloading live filesystem images =====
Mon Dec 23 10:23:26 UTC 2019
2019-12-23 10:29:20 URL:https://launchpadlibrarian.net/456858940/livecd.ubuntu.squashfs [2086760448/2086760448] -> "/home/xnox/canonical/ubuntu-cdimage/scratch/ubuntu/focal/daily-live/live/amd64.squashfs" [1]
No keys found; not signing images.
2019-12-23 10:29:22 URL:https://launchpadlibrarian.net/456858932/livecd.ubuntu.kernel-generic [11399928/11399928] -> "/home/xnox/canonical/ubuntu-cdimage/scratch/ubuntu/focal/daily-live/live/amd64.kernel-generic" [1]
2019-12-23 10:29:24 URL:https://launchpadlibrarian.net/456858933/livecd.ubuntu.kernel-oem [8359576/8359576] -> "/home/xnox/canonical/ubuntu-cdimage/scratch/ubuntu/focal/daily-live/live/amd64.kernel-oem" [1]
2019-12-23 10:29:39 URL:https://launchpadlibrarian.net/456858927/livecd.ubuntu.initrd-generic [86440094/86440094] -> "/home/xnox/canonical/ubuntu-cdimage/scratch/ubuntu/focal/daily-live/live/amd64.initrd-generic" [1]
2019-12-23 10:29:52 URL:https://launchpadlibrarian.net/456858930/livecd.ubuntu.initrd-oem [76189083/76189083] -> "/home/xnox/canonical/ubuntu-cdimage/scratch/ubuntu/focal/daily-live/live/amd64.initrd-oem" [1]

This looks reasonable, but is there a link to a spec for the overall design here?

There is no link to a public spec for the overall design.

At the moment, this is simply about providing more than one kernel flavour on the desktop iso out of the set of [generic, oem, hwe] similar to how on server ISOs we have provided more than one kernel flavour since forever.

There are further pieces to have automatic sensible default flavour selection, installing matching one into target, and installer UX changes. But none of those have been defined yet, as we are currently looking to have an ISO with multiple kernel flavours to prototype with to see how this will look like.