Revert "ubuntu-minimal : only install 16.04 HWE kernel on xenial"

This reverts commit 14ff8f942c076936e79ee2c5fce1e9f34efe55a3.

This seems to not be installing the kernel at all, and needs further
investigation.

Change-Id: Ifd809d4b67aff5d80f979235db246a16af0375b3

diskimage_builder/elements/ubuntu-minimal/package-installs.yaml

@@ -1,37 +1,27 @@
 linux-image-generic:
   not-arch: arm64
   when: DIB_UBUNTU_KERNEL = linux-image-generic
-
-# On ARM64/Xenial install the HWE kernel as generic because the base
-# kernel is very old and causes problems.
-linux-generic-hwe-16.04:
-  arch: arm64
-  when:
-    - DIB_RELEASE = xenial
-    - DIB_UBUNTU_KERNEL = linux-image-generic
-linux-image-generic:
-  arch: arm64
-  when:
-    - DIB_RELEASE != xenial
-    - DIB_UBUNTU_KERNEL = linux-image-generic
-
-# NOTE: there isn't actually an arm64 linux-image-kvm, but we don't
-# have a way to error out here.
 linux-image-kvm:
   not-arch: arm64
   when: DIB_UBUNTU_KERNEL = linux-image-kvm
-
 linux-image-virtual:
+  not-arch: arm64
   when: DIB_UBUNTU_KERNEL = linux-image-virtual
-
 linux-generic-hwe-18.04:
+  not-arch: arm64
   when:
     - DIB_UBUNTU_KERNEL = linux-image-hwe
     - DIB_RELEASE = bionic
-
 linux-generic-hwe-20.04:
+  not-arch: arm64
   when:
     - DIB_UBUNTU_KERNEL = linux-image-hwe
     - DIB_RELEASE = focal
 
+# TODO(ianw): this is Xenial specific, for now.  This kernel works
+# much better, however, including better support for cdrom's for
+# config-drive.  We'll need to filter this further for future
+# releases; maybe add a $DIB_RELEASE filter too?
+linux-generic-hwe-16.04:
+  arch: arm64
 initramfs-tools: