AC_Fence: increase default radius to 300m

300m is still within line of sight and this should reduce the number of
false breaches due to bad GPS positions
AutoTest: reduce fence radius to 150m for testing
AP_Compass: DEVIOCGDEVICEID is (currently) not defined in VRNuttX
AP_HAL: use VRBRAIN INS/BARO/COMPASS for VRBRAIN boards
tools: Make install-prereqs-ubuntu.sh work with newer versions of Ubuntu

* Install libraries for 64bit Ubuntu systems.
* Less noise apt-get output
* Install packages required for SITL simulation
* Install pymavlink, MAVProxy and droneapi
* Remove modemmanager package
* Add autotest/ to the path
tools: In install-prereqs-ubuntu.sh add support for relative paths

Install the required projects (PX4*) in the same folder from where the script is called.
Install the arm toolchain at /opt
tools: Adding support for Travis-CI
Travis-CI: add vrubrain-v51 target (Copter)
autotest: Add code to detect if a hard link is not allowed (for VM's).
HAL_Linux: made I2C driver more portable

this uses a typeof() to cope with different kernel data types for I2C
messages
HAL_Linux: fixed a build warning
HAL_Linux: added bus scanning test