manual: make clean target consistent with others in buildroot

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

docs/manual/manual.mk

@@ -38,10 +38,10 @@ $(call GENDOC_INNER,$(1),chunked,split-html,chunked,Split HTML)
 $(call GENDOC_INNER,$(1),pdf,pdf,pdf,PDF,--dblatex-opts "-P latex.output.revhistory=0")
 $(call GENDOC_INNER,$(1),text,txt,text,Text)
 $(call GENDOC_INNER,$(1),epub,epub,epub,EPUB)
-clean: clean-$(1)
-clean-$(1):
+clean: $(1)-clean
+$(1)-clean:
 	$(Q)$(RM) -rf $(O)/docs/$(1)
-.PHONY: $(1) clean-$(1)
+.PHONY: $(1) $(1)-clean
 endef
 
 MANUAL_SOURCES = $(wildcard docs/manual/*.txt) $(wildcard docs/images/*)