testing: Make strongSwan build recipe more configurable
testing: Add packages to rebuild strongSwan from the repository
testing: Add a script to build the current (or an arbitrary) source tree

This allows to (relatively) quickly (re-)build and install the current
or an arbitrary strongSwan source tree within the root image.

bindfs is used to bind mount the source directory using the regular user
and group (only works if sudo is used to run the script) so that newly
created files are not owned by root.

As with building the root image in general the guests must not be
running while executing this script.  The guest images are automatically
rebuilt after the root image has been updated so configuration files and
other modifications in guests will be lost.
testing: Use multiple jobs to install strongSwan