bluez4: Allow systemd to activate bluetooth

The bluetooth.service runs bluetoothd to manage bluetooth devices, if
present, in the system.  By design, it should be running in either of
two cases:
1. A userspace utility (e.g. networkmanager) has made a dbus request
   for bluetooth services provided by org.bluez.  Even without this
   patch, the bluetooth.service gets run via dbus activation under the
   systmed aliased name 'dbus-org-bluez.service'. Perfect!
2. A bluetooth device is added to the system.  When a bluetooth device
   is added, udev triggers the special systemd target, bluetooth.target
   intended to pull in any services needed for bluetooth linked under
   bluetooth.target.wants.

Enable bluetooth.service so it gets linked under bluetooth.target.wants
and bluetoothd gets started when a user adds a bluetooth device to the
system.

To be clear, this isn't forcing bluetooth to be running all the time---
only when either of #1 or #2 are true.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>