feat: say whether a host is still in its first boot

first_boot_pending() is true while /firstboot exists on FreeBSD. A FreeBSD
cloud image upgrades its base system on its first boot, starts sshd only
after that and restarts right away, and /etc/rc removes /firstboot just
before that restart. netOrk waits for this before it sets up a new VM
(NetOrk/netork#795).

OpenBSD has no such marker to ask yet, so it returns False there.

napalm_bsd/base.py

@@ -188,6 +188,21 @@ class BsdDriver(HostStatusMixin, OSDriver):
         and the readers below treat empty output as "nothing there"."""
         return self.run_command(command, privileged=privileged, timeout=timeout).stdout.strip()
 
+    #: A file that exists until the host's first boot is over; "" when there is none to ask.
+    FIRST_BOOT_MARKER = ""
+
+    def first_boot_pending(self) -> bool:
+        """Whether the host is still in its first boot.
+
+        A FreeBSD cloud image upgrades its base system on the first boot, starts
+        sshd only after that and restarts right away; ``/etc/rc`` removes
+        ``/firstboot`` just before the restart. netOrk waits for this before it
+        sets up a new VM (NetOrk/netork#795).
+        """
+        if not self.FIRST_BOOT_MARKER:
+            return False
+        return self.run_command(f"test -e {self.FIRST_BOOT_MARKER}").exit_code == 0
+
     def _run_host_status_command(self, command: str) -> str:
         """The transport for ``HostStatusMixin.get_host_status``: read-only, no root.