Replace 'path' tags with 'tag-X'

Using path identifiers for tag names causes ambiguity as it's not (immediately) clear whether a reference to it is a path or a tagname. This causes hard to read/interpret log files and can lead to subtle and hard to detect bugs.

The same tag can refer to different things based on the context (partition/device/mountpoint/etc), so just use the 'tag-' prefix.

On the previous 'mount: /' step, I also added that it's to be mounted on "dirname: '/'" to make it (more) explicit. And removed it again as it seems you need to specify both 'dirname' AND 'mount-on' or neither. See https://bugs.debian.org/1023321

Also replace '/boot' tag with 'tag-firmware'.

This was the initial trigger for the tag-rename 'operation' as it caused confusion (with me) as it was also a mislabeled tag, so remove the ambiguity by renaming it to 'tag-firmware'.