[Merge] ~xnox/livecd-rootfs:preinstalled into livecd-rootfs:ubuntu/master

[Dimitri John Ledkov]

Addressing review feedback of making it harder to build unrelated/unexpected images with hardcoded passwords.

1) change auto/config to emit hardcoded_passwords setting & image_target to _just_ disk1-img-xz for just the current preinstalled image targets

2) added guards around adding hardcoded passwords, and fail image build if anything but disk1-img-xz is being built

3) added new image target disk1-img-xz which is _not_ pulled in by default by "all"

This means if anything attempts to pull in disk1-img-xz target and thus building "disk-image" with hardcoded password, no other image format can be produced. And any unexpected combinations of arch/subarch/project will also fail the the build.
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https://code.launchpad.net/~xnox/livecd-rootfs/+git/livecd-rootfs/+merge/401222
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~xnox/livecd-rootfs:preinstalled into livecd-rootfs:ubuntu/master.