rtl-sdr: Add start script file for rtl-tcp

One of the most common uses of the rtl-sdr suite on OpenWrt is to stream
data to a remote analyzer. This commit creates a /etc/init.d/ and
/etc/config entry for rtl-tcp, so that it can automatically launch on
startup.

Signed-off-by: Mike Miller <github@mikeage.net>

utils/rtl-sdr/files/rtl_tcp.config

@@ -0,0 +1,12 @@
+config rtl_tcp main
+	option disabled '1'
+	option respawn '1'
+	option device_index ''
+	option address '0.0.0.0'
+	option port ''
+	option frequency ''
+	option gain ''
+	option samplerate ''
+	option buffers '8'
+	option num_linked_lists '8'
+	option ppm_error ''