ubuntu-cpc: Mask blkid failure in mount_image

On trusty parted does not create the FS based on the type passed to it.
As a result, calling mount_image for a newly created image will not have
an FS on the root partition.  Calling blkid in mount_image to provide
additional debug output will fail when the root partition has no FS.
The root FS is created after mount_image is called in the hook that
creates the base.ext4 image.  We can ignore the blkid failure as it is
used for debug output; the base image will not have output but derivative
images already have a formatted partition and we'll have handy info.

live-build/ubuntu-cpc/functions

@@ -60,7 +60,7 @@ mount_image() {
 
     # Add some information to the debug logs
     echo "Mounted disk image ${backing_img} to ${rootfs_dev_mapper}"
-    blkid ${rootfs_dev_mapper}
+    blkid ${rootfs_dev_mapper} || true
 
     return 0
 }