Better inform the user about nodes' bootup status

[eduble]

Old raspberry pi boards may take a little while for booting (a few minutes).
The experimental hybrid boot feature may also incur much longer boot times when the image is booted for the first time and must be copied to the node's disk.
If we change the walt-init log stream and send more formalized boot status updates, we could then inform the user better:

• by adding no: booting to the possible values of column Booted of walt node show -- this would be printed after we get a first sign of the node's bootup procedure (walt-init log line? register_device() call? walt-server-httpd query?)
• by indicating the current bootup step of remaining nodes in walt node wait (off/early-boot, walt init, hybrid-mode: copying OS, OS init; or at various stages of bootup when relevant)