JWB_SCRIPT_DIR bypasses $(JWBDIR) for its scripts candidate, re-assembling
./$(JWBDIR_NAME)/scripts from the bare name. That works, but is redundant
resolution machinery which should be removed per se.
Resolve the scripts directory from $(JWBDIR) instead. Aside from
simplifying, that fixes the resolution when projects-dir.mk is included
from Makefiles other than projects-dir-miminimal.mk, and complements work
to commit f3f0ef10ca which also aims at allowing to run make from a
bind-mounted dependency closure of repositories.
Signed-off-by: Jan Lindemann <jan@janware.com>