docbook: make cleandocs

Add a 'make cleandocs' target to clean up all generated
DocBook files.
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/DocBook/Makefile

@@ -31,7 +31,7 @@ PS_METHOD	= $(prefer-db2x)
 
 ###
 # The targets that may be used.
-PHONY += xmldocs sgmldocs psdocs pdfdocs htmldocs mandocs installmandocs
+PHONY += xmldocs sgmldocs psdocs pdfdocs htmldocs mandocs installmandocs cleandocs
 
 BOOKS := $(addprefix $(obj)/,$(DOCBOOKS))
 xmldocs: $(BOOKS)
@@ -213,11 +213,12 @@ silent_gen_xml = :
 dochelp:
 	@echo  ' Linux kernel internal documentation in different formats:'
 	@echo  '  htmldocs        - HTML'
-	@echo  '  installmandocs  - install man pages generated by mandocs'
-	@echo  '  mandocs         - man pages'
 	@echo  '  pdfdocs         - PDF'
 	@echo  '  psdocs          - Postscript'
 	@echo  '  xmldocs         - XML DocBook'
+	@echo  '  mandocs         - man pages'
+	@echo  '  installmandocs  - install man pages generated by mandocs'
+	@echo  '  cleandocs       - clean all generated DocBook files'
 
 ###
 # Temporary files left by various tools
@@ -235,6 +236,10 @@ clean-files := $(DOCBOOKS) \
 
 clean-dirs := $(patsubst %.xml,%,$(DOCBOOKS)) man
 
+cleandocs:
+	$(Q)rm -f $(call objectify, $(clean-files))
+	$(Q)rm -rf $(call objectify, $(clean-dirs))
+
 # Declare the contents of the .PHONY variable as phony.  We keep that
 # information in a variable se we can use it in if_changed and friends.