DocBook: use DOCBOOKS="" to ignore DocBooks instead of IGNORE_DOCBOOKS=1

Instead of a separate ignore flag, use the obvious DOCBOOKS="" to ignore
all DocBook files. This is also in line with the Sphinx build being
ignored if a non-empty DOCBOOKS make variable is specified on the make
command line.

This replaces the IGNORE_DOCBOOKS introduced in

commit 547218864afb2745d9d137f005f3380ef96b26ab
Author: Mauro Carvalho Chehab <[e-mail]>
Date: Sat Jul 9 13:12:45 2016 -0300

doc-rst: add an option to ignore DocBooks when generating docs

and aligns with

commit 6387872c86ea6698ed8faa3ccad1d1bd60f762f7
Author: Jani Nikula <[e-mail]>
Date: Fri Jul 1 15:24:44 2016 +0300

Documentation/sphinx: skip build if user requested specific DOCBOOKS

Cc: Daniel Vetter <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Tested-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/DocBook/Makefile

···
 # To add a new book the only step required is to add the book to the
 # list of DOCBOOKS.
 
-ifeq ($(IGNORE_DOCBOOKS),)
-
 DOCBOOKS := z8530book.xml device-drivers.xml \
 	    kernel-hacking.xml kernel-locking.xml deviceiobook.xml \
 	    writing_usb_driver.xml networking.xml \
···
 	    alsa-driver-api.xml writing-an-alsa-driver.xml \
 	    tracepoint.xml gpu.xml media_api.xml w1.xml \
 	    writing_musb_glue_layer.xml crypto-API.xml iio.xml
+
+ifeq ($(DOCBOOKS),)
+
+# Skip DocBook build if the user explicitly requested no DOCBOOKS.
+.DEFAULT:
+	@echo "  SKIP    DocBook $@ target (DOCBOOKS=\"\" specified)."
+
+else
 
 include Documentation/DocBook/media/Makefile
 
···
 	       -e "s/>/\\>/g";     \
 	   echo "</programlisting>")  > $@
 
-else
-
-# Needed, due to cleanmediadocs
-include Documentation/DocBook/media/Makefile
-
-htmldocs:
-pdfdocs:
-psdocs:
-xmldocs:
-installmandocs:
-
-endif # IGNORE_DOCBOOKS
-
+endif # DOCBOOKS=""
 
 ###
 # Help targets as used by the top-level makefile
···
 	@echo  '  make DOCBOOKS="s1.xml s2.xml" [target] Generate only docs s1.xml s2.xml'
 	@echo  '  valid values for DOCBOOKS are: $(DOCBOOKS)'
 	@echo
-	@echo  "  make IGNORE_DOCBOOKS=1 [target] Don't generate docs from Docbook"
+	@echo  "  make DOCBOOKS=\"\" [target] Don't generate docs from Docbook"
 	@echo  '     This is useful to generate only the ReST docs (Sphinx)'