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/rc.local.fast

@@ -0,0 +1,42 @@
+#!/bin/bash
+# rc.local for the OneFinity Pi, "fast" variant.
+#
+# What changed vs. scripts/rc.local:
+#   - bbserial unbind/rebind moved to bbserial-rebind.service (runs
+#     once, before bbctrl, instead of after bbctrl is already
+#     listening on the serial port).
+#   - startx moved to kiosk.service so chromium starts in parallel
+#     with bbctrl rather than blocking on rc.local.
+#   - rc.local no longer keeps the Pi in 'starting' state forever,
+#     which fixes systemd-analyze.
+
+set -e
+
+# Mount /boot read only
+mount -o remount,ro /boot 2>/dev/null || true
+
+# Set SPI GPIO mode
+gpio mode 27 alt3 || true
+
+# Create browser memory limited cgroup
+if [ -d /sys/fs/cgroup/memory ]; then
+    CGROUP=/sys/fs/cgroup/memory/chrome
+    [ -d "$CGROUP" ] || mkdir -p "$CGROUP"
+    chown -R pi:pi "$CGROUP"
+    echo 650000000 > "$CGROUP/memory.soft_limit_in_bytes"
+    echo 750000000 > "$CGROUP/memory.limit_in_bytes"
+fi
+
+# Stop boot splash; harmless if plymouth already gone.
+plymouth quit 2>/dev/null || true
+
+# Start X (chromium kiosk) in the background so rc.local can exit and
+# late-boot units (bbctrl logrotate, etc.) don't block on it. Output
+# is redirected so the journal doesn't fill up with X warnings.
+cd /home/pi
+# `-- -nocursor` hides the X pointer; this is a touchscreen kiosk and
+# the mouse cursor only gets in the way.
+nohup sudo -u pi startx -- -nocursor >/var/log/onefin-x.log 2>&1 &
+disown
+
+exit 0