GPS UART fix for RPi 3

[ghost]

• Resolves RPi3: GPS cannot connect via UART #393 by removing /dev/ttyS0 from GPS initialization sequence. On the RPi 3, this port does not connect to GPIO pins 8 and 10; receivers connected to these pins are therefore not detected.
• Add a interactive script for configuring MTK3339 GPS receivers from the command line

[peepsnet]

See comments in #393 .

[ghost]

@peepsnet -- I added the fix for GPIO UART to config.txt

[peepsnet]

great. Do we know if there are any ill effects with RPi2's and RPi0's???

[peepsnet]

I applied the changes in this PR(for the RY835AI) and compiled locally on my pi2 and pi3 and there were no ill effects.

This is what I did:
nano /boot/config.txt
add dtoverlay=pi3-miniuart-bt

and removed the /dev/ttyS0 reference from ry835ai.go

I do have my 835 GPS functioning now on my pi3!!!!!!!

[ghost]

@peepsnet -- thanks for the test report.

I see the same good results on my Pi2 (ry835ai over GPIO serial) with both an upgraded v0.8r2 Jessie image, and with an older Wheezy image that I've been using.