Refer to target file rather than target artefact for Maven build

Currently we refer to the target artefact in the Maven build. We should instead refer to the target file. This prevents having to update the version of the artefact all the time. We can thus simplify the instructions for moving to a next ESCET version, in the developer documentation.

See also:

• https://wiki.eclipse.org/Tycho/Release_Notes/2.3#Target_files_can_be_specified_directly_now
• https://github.com/eclipse/tycho/blob/master/RELEASE_NOTES.md#target-files-can-be-specified-directly-now
• https://bugs.eclipse.org/bugs/show_bug.cgi?id=571520

Tasks

• Switch to Tycho 2.5.0 to get the required bugfix (see #159 (comment 229068)). See issue #158 (closed).
• Apply patch to use target file rather than target artefact (see #159 (comment 229068)).
• Test the full build launch configuration.
• Test all other Maven launch configurations.
• Test build on Jenkins.
• Update developer documentation by removing the step to update the version number of the target artefact in releng/org.eclipse.escet.configuration/pom.xml.