Here is a proposal for the result of make help for the chain-build
target, what I changed relates with : and prallel builds:
chain-build Build current package in order with
example: make chain-build CHAIN='libwidget libgizmo'
The current package is added to the end of the CHAIN list.
Colons (:) can be used in the CHAIN parameter to define
parallely built package groups.
Packages in a single group will be built in parallel,
and all packages in a group must build successfully and
populate the repository before the next group will begin building.
make chain-build CHAIN='libwidget libaselib : libgizmo
will cause libwidget and libaselib to be build in parallel,
followed by libgizmo and then the current directory package.
If no groups are defined, packages will be built sequentially.