DocBook: make index.html generation less verbose by default

When make htmldocs is called on non-verbose mode, it will still be
verbose with index.html generation for no good reason, printing:

rm -rf Documentation/DocBook/index.html; echo '<h1>Linux Kernel HTML Documentation</h1>' >> Documentation/DocBook/index.html && echo '<h2>Kernel Version: 4.4.0-rc1</h2>' >> Documentation/DocBook/index.html && cat Documentation/DocBook/iio.html >> Documentation/DocBook/index.html

Instead, use the standard non-verbose mode, using:

HTML Documentation/DocBook/index.html

if not called with V=1.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/DocBook/Makefile

···
 
 HTML := $(sort $(patsubst %.xml, %.html, $(BOOKS)))
 htmldocs: $(HTML)
-	$(call build_main_index)
+	$(call cmd,build_main_index)
 	$(call install_media_images)
 
 MAN := $(patsubst %.xml, %.9, $(BOOKS))
···
 
 index = index.html
 main_idx = $(obj)/$(index)
-build_main_index = rm -rf $(main_idx); \
+quiet_cmd_build_main_index = HTML    $(main_idx)
+      cmd_build_main_index = rm -rf $(main_idx); \
 		   echo '<h1>Linux Kernel HTML Documentation</h1>' >> $(main_idx) && \
 		   echo '<h2>Kernel Version: $(KERNELVERSION)</h2>' >> $(main_idx) && \
 		   cat $(HTML) >> $(main_idx)