[ubuntu] [preseed] Don't ask about missing /boot during installation

On ppc64, the installer asks confirmation to install without /boot.
It's not a problem, so add the answer to the preseed configuration.

steps/data/preseed/ubuntu-trusty-preseed.cfg

@@ -31,6 +31,9 @@ d-i partman-md/device_remove_md boolean true
 d-i partman-lvm/confirm boolean true
 d-i partman-lvm/confirm_nooverwrite boolean true
 
+# Don't ask about missing /boot
+d-i partman-auto-lvm/no_boot boolean true
+
 d-i partman-auto-lvm/guided_size string max
 d-i partman-auto/choose_recipe select atomic
 d-i partman/default_filesystem string ext4