Building kernel and world in a poudriere jail

Poudriere Image builds world and kernel.
It appears it can build
firmware image (kernel/world + ports). It now depends what you want and how you want it. It may not serve your purpose in the way its output can be generated.

For example, this tool could be used to test (custom) kernel/world before upgrading an existing system but you may not be able to install/use its image to update the existing system without wiping out already existing confs for services.