xpcc communication: parser: support additional include search paths

In addition to searching in the root xml file's directory for
includes, the -I flag can be used to add other directories to
the include path.

Please not: this currently is not integreated into the
system_design scons tool, thus you will not be able
to use this feature with scons at the moment.
However, if you wanted to you could call the
builder files manually.
xpcc communication: include graph: use common include function and draw nicer lables