configs/releng/airootfs/etc/systemd/system/getty@tty1.service.d/autologin.conf: update ExecStart

Update the drop-in to more closely match getty@.service of systemd 250.

Use example from https://wiki.archlinux.org/title/getty#Virtual_console

configs/releng/airootfs/etc/systemd/system/getty@tty1.service.d/autologin.conf

 [Service]
 ExecStart=
-ExecStart=-/sbin/agetty --autologin root --noclear %I 38400 linux
+ExecStart=-/sbin/agetty -o '-p -f -- \\u' --noclear --autologin root - $TERM