ubuntu-minimal : only install 16.04 HWE kernel on xenial

Only install the HWE kernel by default for Xenial.  This was actually
installing the 16.04 HWE kernel on Bionic by accident, since it seems
to have that package; however it was breaking Focal.

On the other distros, just install the default generic kernel.  Let's
KISS for now if we can ...

Change-Id: I34e27d821fbefe274e7b007f37b0bd34db2e1d26

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

@@ -1,27 +1,37 @@
 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: