CI: install pregenerated SSH keys to speed up boot

As we're in a CI environment there's no need to generate unique SSH keys
on boot.  Installing the pregenerated SSH keys means they don't need to
be generated, which saves a reasonable amount of boot time due to the
lack of entropy if virtio-rng isn't available.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/base.yml

@@ -42,6 +42,8 @@ local_conf_header:
     QEMU_USE_KVM = ""
   perf: |
     CORE_IMAGE_EXTRA_INSTALL += "perf"
+  sshkeys: |
+    CORE_IMAGE_EXTRA_INSTALL += "ssh-pregen-hostkeys"
 machine: unset
 
 target: