The stack trace of python calls that resulted in this exception/failure was:
File: 'do_rootfs', lineno: 17, function: <module>
     0013:    # generate final images
     0014:    create_image(d)
     0015:
     0016:
 *** 0017:do_rootfs(d)
     0018:
File: 'do_rootfs', lineno: 14, function: do_rootfs
     0010:    # generate rootfs
     0011:    create_rootfs(d)
     0012:
     0013:    # generate final images
 *** 0014:    create_image(d)
     0015:
     0016:
     0017:do_rootfs(d)
     0018:
File: '/home/aurele/workspace/yocto/test4/sources/poky/meta/lib/oe/image.py', lineno: 337, function: create_image
     0333:        execute_pre_post_process(self.d, post_process_cmds)
     0334:
     0335:
     0336:def create_image(d):
 *** 0337:    Image(d).create()
     0338:
     0339:if __name__ == "__main__":
     0340:    """
     0341:    Image creation can be called independent from bitbake environment.
File: '/home/aurele/workspace/yocto/test4/sources/poky/meta/lib/oe/image.py', lineno: 315, function: create
     0311:        execute_pre_post_process(self.d, pre_process_cmds)
     0312:
     0313:        self._remove_old_symlinks()
     0314:
 *** 0315:        image_cmd_groups = self._get_imagecmds()
     0316:
     0317:        for image_cmds in image_cmd_groups:
     0318:            # create the images in parallel
     0319:            nproc = multiprocessing.cpu_count()
File: '/home/aurele/workspace/yocto/test4/sources/poky/meta/lib/oe/image.py', lineno: 285, function: _get_imagecmds
     0281:                localdata.setVar('OVERRIDES', '%s:%s' % (type, old_overrides))
     0282:                bb.data.update_data(localdata)
     0283:                localdata.setVar('type', type)
     0284:
 *** 0285:                cmds.append("\t" + localdata.getVar("IMAGE_CMD", True))
     0286:                cmds.append(localdata.expand("\tcd ${DEPLOY_DIR_IMAGE}"))
     0287:
     0288:                if type in cimages:
     0289:                    for ctype in cimages[type]:
Exception: TypeError: cannot concatenate 'str' and 'NoneType' objects