boot: cold-boot optimisations cutting bbctrl listen by ~8s on the Pi

- scripts/rc.local.fast: minimal rc.local that defers the heavy bits.
- scripts/bbserial-rebind.service: oneshot unit that unbinds ttyAMA0
  from pl011 and (re)loads bbserial before bbctrl.service.
- scripts/bbctrl.service: declare the After/Wants on bbserial-rebind
  so we can rely on it rather than racing rc.local.
- scripts/install.sh: ship the cold-boot bits with firmware updates
  (mask sysstat, replace dphys-swapfile with an fstab swap entry).
- scripts/rc.local + setup_rpi.sh + setup.py: wire updated paths.

scripts/setup_rpi.sh

@@ -75,7 +75,7 @@ sed -i 's/^PARTUUID=.*\//\/dev\/mmcblk0p2 \//' /etc/fstab
 
 # Enable browser in xorg
 sed -i 's/allowed_users=console/allowed_users=anybody/' /etc/X11/Xwrapper.config
-echo "sudo -u pi startx" >> /etc/rc.local
+echo "sudo -u pi startx -- -nocursor" >> /etc/rc.local
 cp /mnt/host/xinitrc /home/pi/.xinitrc
 cp /mnt/host/ratpoisonrc /home/pi/.ratpoisonrc
 cp /mnt/host/xorg.conf /etc/X11/