add systemd unit file

This can be used to start bootmac reliably (in my limited testing), the
1 second delay seems to be enough for the hw to be ready for setting the
mac addr. There are probably better ways to do this.

Being part of the bluetooth.target means that this service (if
enabled) is started automatically every time the bluetooth service is
(re)started. It's functionally the same as starting bootmac via udev,
except we can easily add delays in this unit file (while delaying udev
rule execution is... dice-y)

bootmac.service

+[Unit]
+Description=Set Mac Address
+PartOf=bluetooth.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStartPre=/bin/sleep 1
+ExecStart=/usr/bin/bootmac --bluetooth
+
+[Install]
+WantedBy=bluetooth.target