CI: use pregenerated SSH keys in genericarm64

We boot genericarm64 inside a qemu, so add the pregenerated keys to speed
up testing.  This isn't a risk because we don't publish the images.

Signed-off-by: Ross Burton <ross.burton@arm.com>

ci/genericarm64.yml

@@ -14,5 +14,8 @@ local_conf_header:
   bootloader: |
     # If running genericarm64 in a qemu we need to manually build the bootloader
     EXTRA_IMAGEDEPENDS += "virtual/bootloader"
+  sshpregen: |
+    # Allow the use of the pregen keys as this is CI so safe
+    COMPATIBLE_MACHINE:pn-ssh-pregen-hostkeys:genericarm64 = "genericarm64"
 
 machine: genericarm64