Recreate symbolic link if it's missing after a sysupgrade with a private and public key present in /etc/atlas/

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>

- Exit start if a probe_key is not present
- Add create_key command to generate a private_key based on the provided username in the atlas config.
- Add registration instruction in /etc/atlas
- Rework script to save probe_key on sysupgrade (the key are now adviced to be placed in the /etc/atlas dir and a link is used to make them accessible in the atlas-sw-scripts etc dir)

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>

Signed-off-by: Jan Pavlinec <jan.pavlinec@nic.cz>