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/bbctrl.service

@@ -1,13 +1,24 @@
 [Unit]
 Description=Buildbotics Controller
-After=network.target
+# Note: bbctrl previously had `After=network.target`. That delays
+# start by ~5s on this Pi while dhcpcd brings up wlan0/eth0, but
+# bbctrl does not actually require network connectivity to come up
+# (the AVR is on a local serial port, the LCD on I2C). Dropping it
+# means the Pi shows the UI faster on cold boot. The wifi config UI
+# still works because it queries iw/dhcpcd lazily on demand.
+After=local-fs.target bbserial-rebind.service
+Wants=bbserial-rebind.service
 
 [Service]
 User=root
 ExecStart=/usr/local/bin/bbctrl -l /var/log/bbctrl.log
 WorkingDirectory=/var/lib/bbctrl
 Restart=always
-StandardOutput=null
+# StandardOutput was 'null'. Set to 'journal' so TRACE lines emitted by
+# bbctrl.Trace are visible via `journalctl -u bbctrl`. Bbctrl still
+# writes its own log via -l above; this only affects stdout/stderr.
+StandardOutput=journal
+StandardError=journal
 Nice=-10
 KillMode=process