Improved hciuart service condition (#7)

The pi3-disable-bt overlay doesn't (and can't) delete the bt_pins node,
but the latest version of the overlay makes its properties (including
`brcm,pins`) zero length. Take advantage of this change by making the
hciuart execution condition:
```
ConditionFileNotEmpty=/proc/device-tree/soc/gpio@7e200000/bt_pins/brcm,pins
```