Simplify creation of custom karaf distributions

From users mailing list.

Question:
How to create custom karaf distribution?

Answer from Gert Vanthienen:

"The easiest way to do this is by creating a features descriptor that
contains a list of all the bundles you need for your application. You
can then use the features-maven-plugin add-features-to-repo goal to
copy all the required bundles into a folder. Once that is done, you
can use the Maven assembly plugin to package everything up.

You can find examples on how to use this in
http://svn.apache.org/viewvc/servicemix/smx4/nmr/trunk/assembly/ and
http://svn.apache.org/viewvc/servicemix/smx4/features/trunk/assembly/.
Looking at the pom files myself now, I guess we really want to put
some effort in making this a little bit easier (either by documenting
it properly or by providing a Maven archetype or plugin) – could you
raise a JIRA issue for this?"