GPS fixes and GPS soft serial support

Added possibility to use both Serial RX (Spektrum satellite/SBUS/SUMD) and GPS at the same time.
- When features: feature SERIALRX, feature SOFTSERIAL and feature GPS are enabled Serial RX is using UART and pins 3&4 and GPS is using soft serial and pins 5&6.
- Fixed GPS code, so that now it's reconnecting if connection to GPS is lost.
- GPS tab on Chrome Configurator now shows GPS signal strength.
- Possibility to disable SBAS (ground assistace type). (SBAS causes occasional severe altitude calcuation errors)
Actually GPS should never lock up or lose connection like that - this was
added strictly for lousy hardware where there's a wire coming loose or
something like that.

On Mon, Nov 24, 2014 at 8:47 PM, Jaakko Haavisto <notifications@github.com>
wrote:

> In src/gps.c:
>
> > @@ -269,13 +330,15 @@ void gpsThread(void)
> >
> >          case GPS_LOSTCOMMS:
> >              gpsData.errors++;
> > -            // try another rate
> > -            gpsData.baudrateIndex++;
> > -            gpsData.baudrateIndex %= GPS_INIT_ENTRIES;
> > +            // try another rate (Removed because if connection is lost we want reconnect, not lower the baud rate)
> > +            //gpsData.baudrateIndex++;
> > +            //gpsData.baudrateIndex %= GPS_INIT_ENTRIES;
>
> Baudrate search is OK if it's made once at start, but if it's made like
> this then it doesn't work or it's confusing.
> It lowers baud rate every time gps serial connection is lost (or GPS
> jams/boots. UBLOX MAX-7 seems to do that it in every 5 to 15 minutes). Just
> reconnect is enough when just connection is lost. Also if it lowers the
> baudrate and there's no way to see it. User is believing that GPS is
> working at 115kbit (as configurator configuration tab shows), but it could
> be working as 19200 rate.
>
> —
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> <https://github.com/multiwii/baseflight/pull/206/files#r20785109>.
>