arm64: use HWE kernel and fix console

Install hwe kernel for ubuntu-minimal.  As noted this is currently
Xenial specific; we need this for initial bring-up so let's tackle
future releases as things progress.

Ensure we use ttyAMA0 for arm64 console too.

Change-Id: Ic607cf8369666dc24929aff6f2ef8a72e7980599

diskimage_builder/elements/bootloader/finalise.d/50-bootloader

@@ -177,6 +177,8 @@ function install_grub2 {
     # Serial console on Power is hvc0
     if [[ "powerpc ppc64 ppc64le" =~ "$ARCH" ]]; then
         SERIAL_CONSOLE="hvc0"
+    elif [[ "arm64" =~ "$ARCH" ]]; then
+        SERIAL_CONSOLE="ttyAMA0,115200"
     else
         SERIAL_CONSOLE="ttyS0,115200"
     fi

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

@@ -1,2 +1,9 @@
 linux-image-generic:
+  not-arch: arm64
+# 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: