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)
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.