Separate XPCC system_tools from :build:scons module

[salkinium]

This adds the submodule :communication:xpcc:generator which copies the tools/system_design python tools to generating the XPCC communication.

Before the :build:scons module copied these files iff the :build:scons:xpcc.source option was set, which was weird when using a custom SConscript, and having to set this to a dummy.xml to get these files copied.

This also fixes duplicate compilation of the generated XPCC sources.

• Rename the system_design folder to something more xpcc related, xpcc_generator?
• Rename the communication examples to xpcc.

cc @dergraaf @rleh @mhthies @chris-durand

[salkinium]

I'm happy with this.

[rleh]

But the CI job unittests is unhappy:

python2: can't open file 'tools/system_design/builder/system_layout.py': [Errno 2] No such file or directory

[salkinium]

But the CI job unittests is unhappy

Too much renamin', too little ack-grep'in.

[salkinium]

fatal error: error writing to /tmp/ccpawS2W.s: Cannot allocate memory

CircleCI sometimes fails to allocate enough memory.

Too many levels of symbolic links

And GitlabCI is weird too.

Le Sigh…