Fix building of primary bootstrap when versions have changed

If the versions of kernel or stdlib have changed, attempting
to update the primary bootstrap (using "./otp_build update_primary")
will fail during the rebuild of the start scripts.

The problem is that the app files in bootstrap/lib/*/ebin/*.app
will not be refreshed if they exist, because of missing dependencies.
Ultimately the dependencies should be fixed, but since there
already is a bootstrap_clean target to work around missing
dependencies, fix bootstrap_clean to also remove
bootstrap/lib/*/ebin/*.app.

Makefile.in

@@ -1023,6 +1023,7 @@ bootstrap_root_clean:
 # $(BOOTSTRAP_TOP) for some reason should be empty).
 bootstrap_clean:
 	rm -f $(ERL_TOP)/bootstrap/lib/*/ebin/*.beam
+	rm -f $(ERL_TOP)/bootstrap/lib/*/ebin/*.app
 	rm -f $(ERL_TOP)/bootstrap/lib/*/egen/*
 	rm -f $(ERL_TOP)/bootstrap/lib/*/include/*.hrl
 	rm -f $(ERL_TOP)/bootstrap/primary_compiler/ebin/*