[Merge] ~waveform/livecd-rootfs:fix-swapfile into livecd-rootfs:ubuntu/master

[Dave Jones]

Dave Jones has proposed merging ~waveform/livecd-rootfs:fix-swapfile into livecd-rootfs:ubuntu/master.

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

For more details, see:
https://code.launchpad.net/~waveform/livecd-rootfs/+git/livecd-rootfs/+merge/415625

The swap file on the Raspberry Pi pre-installed desktop images is broken again, due to some change (either in livecd or ubuntu-image -- I'm suspecting the latter at the moment) copying the pre-allocated swap file such that it winds up with "holes" (and therefore invalid as a swap file). Rather than constantly play whack-a-mole with this bug, we should just allocate the swap file when it doesn't exist.

This MP does this by adding a couple of systemd units:
* mkswap.service to create the swap file when it doesn't exist
* swapfile.swap to activate the swapfile

It also:
* Removes the swapfile entry from fstab in favour of swapfile.swap (makes it much easier to control the timing of mkswap.service to ensure it comes before swapfile.swap)
* Tidies up the manipulation of the fstab (in a similar manner to my other MP) to ensure we remove the "UNCONFIGURED FSTAB" warning which was still lying around, and add the "discard" option (given people tend to install this on SSDs).
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~waveform/livecd-rootfs:fix-swapfile into livecd-rootfs:ubuntu/master.
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