Allow to force the platform from the commandline

[meekrosoft]

Dependent on issue #3

The current build command default is use the buildCmd.<osFamily> but we would like to be able to overwrite this from the command line. There are cross-compile use cases where we want to specify the command for the target rather than host.

./gradlew -PtargetPlatform=raspberrypi

Which for instance would then invoke this build command from the build.properties:

buildCmd.raspberrypi=./build-rpi.sh

http://stackoverflow.com/questions/34875637/how-to-pass-multiple-parameters-in-command-line-when-running-gradle-task
http://stackoverflow.com/questions/11696521/how-to-pass-arguments-from-command-line-to-gradle