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/bbserial-rebind.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=Unbind ttyAMA0 from pl011 and reload bbserial
+DefaultDependencies=no
+After=systemd-modules-load.service local-fs.target
+Before=bbctrl.service
+ConditionPathExists=/sys/bus/amba/drivers/uart-pl011
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+# Tolerate the device already being bound elsewhere or the module
+# already being loaded — the goal is the end state (bbserial owns
+# ttyAMA0), not running the steps.
+ExecStart=/bin/sh -c '\
+    echo 3f201000.serial > /sys/bus/amba/drivers/uart-pl011/unbind 2>/dev/null || true; \
+    /sbin/modprobe -r bbserial 2>/dev/null || true; \
+    /sbin/modprobe bbserial \
+'
+
+[Install]
+WantedBy=multi-user.target