gps file dump: re-implement with an uORB topic & write to the log file (#4987)

Drawbacks of the previous method: when writing to the SD card, there are
high delays in the write() call of several 100ms, every now and then. The
frequency and length of these events depend on:
- SD card
- used logger bandwidth
- bandwidth of gps data (RTCM)
Since the whole gps thread was blocked during this period, it lead to
gps timeouts and lost module.

What we do now is: publish an orb topic with queuing. This makes it async
and the logger takes care of buffering. This means it's best to:
- use high logger rate
- use large logger buffer
- reduce logger bandwith by disabling unused topics

src/drivers/gps/params.c

@@ -34,10 +34,8 @@
 /**
  * Dump GPS communication to a file.
  *
- * If this is set to 1, all GPS communication data will be written to a
- * file. Two files will be created, for reading and writing. All
- * communication from startup until device disarm will be dumped.
- *
+ * If this is set to 1, all GPS communication data will be published via uORB,
+ * and written to the log file as gps_dump message.
  * @min 0
  * @max 1
  * @value 0 Disable