Add systemd unit file

Being part of the bluetooth.target means that this service (if
enabled) is started automatically every time the bluetooth service is
(re)started.

bootmac-bluetooth.service

+[Unit]
+Description=Set Mac Address
+PartOf=bluetooth.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/bin/bootmac --bluetooth
+
+[Install]
+WantedBy=bluetooth.target