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