adb: Add missing scripts and service for adb mode

To get a working adb mode, we need to add the functionfs mounting,
as just starting adbd is not enough anymore.
Signed-off-by: Philippe De Swert <philippedeswert@gmail.com>

systemd/adbd-functionfs.sh

+#!/bin/sh
+set -e
+mkdir -p /dev/usb-ffs
+chmod 0770 /dev/usb-ffs
+chown shell:shell /dev/usb-ffs
+mkdir -p /dev/usb-ffs/adb
+chmod 0770 /dev/usb-ffs/adb
+chown shell:shell /dev/usb-ffs/adb
+/bin/mount -t functionfs adb /dev/usb-ffs/adb -o uid=shell,gid=shell
+exit 0

systemd/adbd-prepare.service

+# Copyright (C) 2013 Jolla Oy
+#
+# Prepare functionfs for adbd
+
+[Unit]
+Description=functionfs setup for adbd
+Before=adbd.service
+PartOf=adbd.service
+
+[Service]
+Type=notify
+RemainAfterExit=yes
+ExecStart=/usr/sbin/adbd-functionfs.sh
+ExecStop=/bin/umount adb
+
+[Install]
+WantedBy=graphical.target