Add explaination to why ubuntu-image == device image.

live-build/ubuntu-cpc/hooks.d/chroot/999-ubuntu-image-customization.chroot

@@ -3,6 +3,11 @@
 . /root/config/common
 
 # Specific ubuntu-image chroot configuration goes here.
+# Currently the ubuntu-image IMAGEFORMAT is equivalent to building an image
+# for a physical device, not a cloud instance. For such images we want some
+# special cloud-init configuration that should not be happening for cloud
+# cases. This check should be changed to a per-MODEL/per-SUBARCH check once
+# we support building cloud images via ubuntu-image.
 if [ "$IMAGEFORMAT" == "ubuntu-image" ]; do
     # If the cloud does not provide a meta-data service this should be run
     # This will setup a nocloud datasource on the boot partition.