Add DIB_UBUNTU_KERNEL to ubuntu-minimal

This patch adds a new environment variable to the ubuntu-minimal
element called DIB_UBUNTU_KERNEL that allows you to specify the kernel
meta package that will be using to install the kernel inside the image.
It supports "linux-image-generic" (The default), "linux-image-kvm", and
"linux-image-virtual".
This allows building images that are smaller in size (~200MB smaller
qcow2) that have only the kernel modules necessary for virtual
machines.

Change-Id: I8ce65e1d357d15e8ed8995ad1dcaea02bbd1986f

diskimage_builder/elements/ubuntu-minimal/README.rst

@@ -14,4 +14,17 @@ be created, the variable ``DIB_RELEASE`` can be set appropriately.
 Note that this element installs ``systemd-sysv`` as the init system for
 18.04+.
 
+Environment Variables
+---------------------
+
+DIB_UBUNTU_KERNEL:
+  :Required: No
+  :Default: ``linux-image-generic``
+  :Description: Specifies the kernel meta package to install in the image.
+  :Example: ``DIB_UBUNTU_KERNEL=linux-image-kvm``
+  :Options: ``linux-image-generic``, ``linux-image-kvm``,
+            ``linux-image-virtual``
+  :Notes: The element must know about the package, otherwise it will select
+          the default.
+
 .. element_deps::

diskimage_builder/elements/ubuntu-minimal/environment.d/11-ubuntu-kernel.bash

@@ -0,0 +1,15 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+#
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+export DIB_UBUNTU_KERNEL=${DIB_UBUNTU_KERNEL:-"linux-image-generic"}

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

@@ -1,5 +1,12 @@
 linux-image-generic:
   not-arch: arm64
+  when: DIB_UBUNTU_KERNEL = linux-image-generic
+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
 # 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

releasenotes/notes/Add-DIB-UBUNTU-KERNEL-5f75a809f3ce9bab.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Added an environment variable, DIB_UBUNTU_KERNEL, that allows you to
+    specify the kernel meta package to install in the image.
+    It currently supports "linux-image-generic" (The default),
+    "linux-image-kvm", and "linux-image-virtual".