Fix for DS-1867 - Maven build issues from [src]/dspace/

This fix ensures the 'native2ascii' call uses 'root.basedir' and not 'basedir' (whose value changes based on which submodule you are running 'mvn' from).  This will ensure 'native2ascii' will also work properly when 'mvn' is called  [src]/dspace/.

This PR also some minor cleanup of the POM, based on the best practice docs for using 'maven-antrun-plugin':
https://maven.apache.org/plugins/maven-antrun-plugin/usage.html

I've tested this, and running "mvn clean" followed by "mvn package" now seems to work fine from *both* [src] and [src]/dspace/.  However, we may wish to have others also verify it is working, just to ensure I haven't overlooked anything.