Outputted root-image.gz has changes from the upstream source

https://bugs.launchpad.net/curtin/+bug/1640519 is kind of the inverse of this bug. On the ARM images u-boot-tools is being added to the root-image.gz but not the SquashFS image. This is causing deployment failures. Until we properly solve that bug we'll want to keep u-boot-tools in for older releases.

Copying your paste here as that is not a public pastebin. Also, not sure what
it is, but there is duplicate data in the output, so I just removed the dupe
lines.

The full set of changes below. The short summary is:
 * apt-get clean
   this runs on all arch, should not be necessary but is not likely harmful.
   we can/should change it to run only if it made apt changes.

 * symlink ENI (bug 1534205)
   not necessary any more for xenial.

 * on armhf and arm64 we install u-boot-tools
   after bug 1640519 is fixed in curtin in all releases we can remove this.
   the code now lists that bug in its change comment.

=== xenial-amd64 ===
CHANGE: apt-get clean
CHANGE: symlink ENI for cloud-initramfs-dyn-netconf (LP: #1534205)

=== xenial-s390x ===
CHANGE: apt-get clean
CHANGE: symlink ENI for cloud-initramfs-dyn-netconf (LP: #1534205)

=== xenial-arm64 ===
CHANGE: install missing packages [u-boot-tools ]
CHANGE: run apt-update [install].
CHANGE: apt-get clean
CHANGE: symlink ENI for cloud-initramfs-dyn-netconf (LP: #1534205)
CHANGE: package changes: add=u-boot-tools remove=

=== xenial-armhf ===
CHANGE: install missing packages [u-boot-tools ]
CHANGE: run apt-update [install].
CHANGE: apt-get clean
CHANGE: symlink ENI for cloud-initramfs-dyn-netconf (LP: #1534205)
CHANGE: package changes: add=u-boot-tools remove=

=== xenial-powerpc ===
[doesnt build maas image]

=== xenial-i386 ===
CHANGE: apt-get clean
CHANGE: symlink ENI for cloud-initramfs-dyn-netconf (LP: #1534205)

=== xenial-ppc64el ===
CHANGE: apt-get clean
CHANGE: symlink ENI for cloud-initramfs-dyn-netconf (LP: #1534205)