The canonical hint file `.vyatta_config` is used by the image tools when searching for previous installations. Traditionally, (hence the file name) this hint was added by the install image scripts; this practice was carried over in the revised image tools. https://vyos.dev/T8265 explicitly added the hint to the raw image creation (now independent of the install image scripts, hence lacking that mechanism); however, this poses its own problems. Drop the 'traditional' method as no longer appropriate, in favor of simple addition in the vyos-1x.postinst script. Note that this is now reasonable after rationalization of the canonical config path (https://vyos.dev/T7836).