Generate feature maven projects

Instead of generating the feature directly. This is neccessary for
p2-maven-plugin to find the resulting features via maven to include
them in the p2 update site.

This commit also cleans up the properties for the script a bit, there
now is a build directory, into which the parent pom is generated and
automatically generates features in target/generated/features and the
p2 maven project pom into target/generated/p2-repo.
Remove destinationDirectory

This property is broken for p2-maven-plugin.