argument parsing fails when using `--foo=bar` instead of `--foo bar`

[Finkregh]

when using = in arguments the argument parsing breaks:

does not work:
kiwi --profile=vmware system boxbuild --box leap -- --description=descriptions/sles-15sp2-amd64-vmware --target-dir=out/sles-15sp2-amd64-vmware

does work:
kiwi --profile=vmware system boxbuild --box leap -- --description descriptions/sles-15sp2-amd64-vmware --target-dir out/sles-15sp2-amd64-vmware

error message:

[ INFO    ]: 13:49:38 | Building with:
    ['--type', 'oem', '--profile', 'vmware', 'system', 'build', '--description=descriptions/sles-15sp2-amd64-vmware', '--target-dir=out/sles-15sp2-amd64-vmware']
[ ERROR   ]: 13:49:38 | Unexpected error:
Traceback (most recent call last):
  File "/usr/local/bin/kiwi", line 8, in <module>
    sys.exit(main())
  File "/opt/venv/lib/python3.9/site-packages/kiwi/kiwi.py", line 68, in main
    App()
  File "/opt/venv/lib/python3.9/site-packages/kiwi/app.py", line 35, in __init__
    task_class().process()
  File "/opt/venv/lib/python3.9/site-packages/kiwi_boxed_plugin/tasks/system_boxbuild.py", line 160, in process
    box_build.run(
  File "/opt/venv/lib/python3.9/site-packages/kiwi_boxed_plugin/box_build.py", line 76, in run
    desc = self._pop_arg_param(
  File "/opt/venv/lib/python3.9/site-packages/kiwi_boxed_plugin/box_build.py", line 155, in _pop_arg_param
    arg_index = self.kiwi_build_command.index(arg)
ValueError: '--description' is not in list