Fixed check for the existence of previous error log files.

The '-f' parameter of test ie '[' expects a single file and seems to
error out on wildcards.
Solution found at
http://stackoverflow.com/questions/6363441/check-if-a-file-exists-with-wildcard-in-shell-script
Added code which installs libraspberrypi-bin from the raspbian archive.

This adds the 'vcgencmd' and friends to the system.
Done this by creating a apt preferences file and installing the package.
Fixes issue #65.