Makefile: declare 'help' target as phony.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -434,7 +434,7 @@ prepare: $(BUILD_DIR)/buildroot-config/auto.conf
 world: target-post-image
 
 .PHONY: all world toolchain dirs clean distclean source outputmakefile \
-	legal-info legal-info-prepare legal-info-clean printvars \
+	legal-info legal-info-prepare legal-info-clean printvars help \
 	target-finalize target-post-image \
 	$(TARGETS) $(TARGETS_ROOTFS) \
 	$(TARGETS_DIRCLEAN) $(TARGETS_SOURCE) $(TARGETS_LEGAL_INFO) \