tools/bpf: Always run the *-clean recipes

Make $(LIBBPF)-clean and $(LIBBPF_BOOTSTRAP)-clean .PHONY targets, in
case those files exist. And keep consistency within the Makefile by
making the directory dependencies order-only.
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201112091049.3159055-2-jean-philippe@linaro.org

tools/bpf/bpftool/Makefile

@@ -44,11 +44,11 @@ $(LIBBPF_BOOTSTRAP): FORCE | $(LIBBPF_BOOTSTRAP_OUTPUT)
 	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_BOOTSTRAP_OUTPUT) \
 		ARCH= CC=$(HOSTCC) LD=$(HOSTLD) $@
 
-$(LIBBPF)-clean: $(LIBBPF_OUTPUT)
+$(LIBBPF)-clean: FORCE | $(LIBBPF_OUTPUT)
 	$(call QUIET_CLEAN, libbpf)
 	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_OUTPUT) clean >/dev/null
 
-$(LIBBPF_BOOTSTRAP)-clean: $(LIBBPF_BOOTSTRAP_OUTPUT)
+$(LIBBPF_BOOTSTRAP)-clean: FORCE | $(LIBBPF_BOOTSTRAP_OUTPUT)
 	$(call QUIET_CLEAN, libbpf-bootstrap)
 	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_BOOTSTRAP_OUTPUT) clean >/dev/null