CleanFlight port to PI Zero

Finally have MPU-9255 and MS5611 sensors working and a simple CLI on the command line.  It is simlar to CLI via serial port or CleanFlight configurator.

Here is the output:
linuxflight>tasks
Task list          max/us  avg/us rate/hz maxload avgload     total/ms
 0 -       SYSTEM      29       0       9    0.5%    0.5%         0
 1 -     GYRO/PID     748     145     848   63.9%   12.7%       743
 3 -       SERIAL      71       1      93    1.1%    0.5%         1
 5 -           RX       0       0   2147483647    0.5%    0.5%         0
linuxflight>tasks
Task list          max/us  avg/us rate/hz maxload avgload     total/ms
 0 -       SYSTEM      31       0       9    0.5%    0.5%         1
 1 -     GYRO/PID     886     176     801   71.4%   14.5%      5864
 3 -       SERIAL      73       1      88    1.1%    0.5%         9
 5 -           RX       0       0   2147483647    0.5%    0.5%         0
linuxflight>

I'm going to switch to linenoise for controlling line processing to allow up and down errors etc. 

Need to fix how average is calculated, but making progress.  Need to reconnect the I/O processor to get GPS, pwm out, s.bus in, and S.Port.

The goal is to use socat where configurator is running to expose a virtual serial port and forward the packets to "LinuxFlight".

Still working on CPU performance and getting GPIO to trigger for gyro-sync.

Here is top:

Here is the CLI on the PI Zero:

The command line is using boost ASIO.  Goal is to add the socat protocol so configurator can still access and configure "LinuxFlight" ;)

Yes, I'm thinking of forking the code, due to the amount of changes to the core.  Right now, there are very minimal.