Use stdin in example direwolf.conf to be compatible with rtl-sdr piped input

files/boot/direwolf.conf

@@ -1,6 +1,4 @@
-# We might not have an audio output device so set second param to null.
-# null input will be overwritten on the command line if not disabling SDR
-ADEVICE null null
+ADEVICE stdin null
 ARATE 24000
 CHANNEL 0