flua is now built as a bootstrap tool for older branches/releases to have a path to generating sysent without having to install flua that does not yet appear there. The compromise is that one must have built bootstrap tools before running sysent, and only the top-level sysent target can be used. This should not be a major problem, though- running all sysent targets does not take long, and this seems to be even more true in the flua world.
makesyscalls.sh must still remain in-tree to allow time for downstream consumers to adjust.