Implement conditionals and variables in package xml
Add support for expanding environment variables at build time for package xml files. In addition, it is now possible to add the 'Condition' attribute to package.xml elements. An element will be removed at create time if the condition evaluates to false.
Using this new feature, merge the 3 existing package.xml files into a single file.
Closes #683
Closes #634
Edited by Alexander Nørskov Larsen