testing: Add option to build all software recipes when building strongSwan

testing: Add option to build all software recipes when building strongSwan

This is like building the root image but using a specific strongSwan
source tree, which is helpful if code changes depend on other software
packages (e.g. TKM-related or testing new crypto libraries).  If the script
is called and the root image does not exist, the new option is enabled
automatically.

The option to build in a specific guest image is now also moved to an
explicit command line option so that the source dir path is the only
remaining positional argument (see --help for details).