Format specifiers for IETF system hostname and password

[troglobit]

As discussed in #326, generation of parts of factory-config are a bit opaque to users. An improvement would be to introduce a more declarative syntax:

• /system/hostname: Support %-specifiers. We should probably support at least:
  • %i: OS ID (from /etc/os-release). infix on vanilla builds,
  • %m: NIC specific part of base MAC. c0:ff:ee
• /system/authentication/user/password: Augment model to allow the literal factory-default or similar. This would indicate that the device's factory password, whatever it may be, should be used.

There has been requests for supporting yescrypt, and similar, so updating the password hash pattern in ietf-system for $y$ will open up for also carrying a string like factory-default as well.

This would make it easier to support static factory-configs, though there are still problems to solve there I think (SSH server keys come to mind).