When kill(-1) returns ESRCH, it could be because the current process

doesn't have a process group, which can occur if you're working with
a custom init that doesn't set up a full tty context.  Rather than
refusing to reboot, ignore ESRCH from the kill attempt in reboot(8).

sbin/reboot/reboot.c

@@ -164,7 +164,7 @@ main(int argc, char *argv[])
 	(void)signal(SIGHUP, SIG_IGN);
 
 	/* Send a SIGTERM first, a chance to save the buffers. */
-	if (kill(-1, SIGTERM) == -1)
+	if (kill(-1, SIGTERM) == -1 && errno != ESRCH)
 		err(1, "SIGTERM processes");
 
 	/*