waf: ardupilotwaf: reset LAST_IDX on every build

That is a workaround so that one can use `waf bin tests`, for example.  In our
context, the only restriction to the task generators creation is values defined
during configuration. Thus, ideally, the recursion and task generators creation
should be done only once for multiple build calls. We should do that in the
future.

Tools/ardupilotwaf/ardupilotwaf.py

@@ -317,5 +317,10 @@ def options(opt):
     )
 
 def build(bld):
+    global LAST_IDX
+    # FIXME: This is done to prevent same task generators being created with
+    # different idx when build() is called multiple times (e.g. waf bin tests).
+    # Ideally, task generators should be created just once.
+    LAST_IDX = 0
     bld.add_pre_fun(_process_build_command)
     bld.add_pre_fun(_select_programs_from_group)