flua: fix buildworld from a clean room

now that the flua ucl module is built the lib directory, it is being
build at a moment where it cannot link yet to libucl, push libucl in
the _prebuild_libs to ensure it is present in a path to be linked
against at the time the lua ucl module is being built.

While here, remove libucl from boostrap as a dependence of flua as it is
not needed anymore now that flua ucl module is dynamically loadable.