USART fixes based on feedback from @ledvinap.
Avoid use of calls to the display when display not supported by the
target.
Minor usart code readability improvement.
GPS - Update UBOX auto-configure to use Pedestrian navigation mode.

"Applications with low acceleration and speed, e.g. how a pedestrian
would move. Low acceleration assumed. MAX Altitude [m]: 9000, MAX
Velocity [m/s]: 30, MAX Vertical Velocity [m/s]: 20, Sanity check type:
Altitude and Velocity, Max Position Deviation: Small"
DISPLAY - Only compile in GPS page if GPS is enabled.
DISPLAY - Skip GPS page if GPS is not enabled.
UART1 - Fix interrupt race condition which resulted in UART randomly
stopping transmission of data.
GPS - Add satellite strength bargraphs to OLED display.
Merge branch 'master' of https://github.com/hydra/cleanflight

Conflicts:
	src/main/drivers/serial_uart.h