ref-manual, Makefile: New section on BitBake for expanded build

Fixes [YOCTO #2808]

Created a new section called "BitBake" that will provide an
expanded discussion of the basic YP build process.  The
"BitBake" section has a new sub-section called "Source Fetching."

Part of the new section is a new figure named source-fetching.png.
The figure must reside in the "figures" directory in both the
ref-manual and the mega-manual.  It must also be listed for each
of these manuals in the TARBALL statement in the Makefile.

(From yocto-docs rev: 41eff3e755a98ff31cadccfb1cd7dfcf569b4d9e)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/Makefile

@@ -198,7 +198,8 @@ TARFILES = mega-manual.html mega-style.css figures/yocto-environment.png figures
         figures/sched-wakeup-profile.png figures/sysprof-callers.png \
         figures/sysprof-copy-from-user.png figures/sysprof-copy-to-user.png figures/cross-development-toolchains.png \
 	figures/yocto-environment-ref.png figures/user-configuration.png figures/source-input.png \
-	figures/package-feeds.png figures/layer-input.png figures/images.png figures/sdk.png
+	figures/package-feeds.png figures/layer-input.png figures/images.png figures/sdk.png \
+	figures/source-fetching.png
 	endif
 
 MANUALS = $(DOC)/$(DOC).html
@@ -215,7 +216,7 @@ TARFILES = ref-manual.html ref-style.css figures/poky-title.png \
         figures/cross-development-toolchains.png figures/layer-input.png \
 	figures/package-feeds.png figures/source-input.png \
 	figures/user-configuration.png figures/yocto-environment-ref.png \
-	figures/images.png figures/sdk.png
+	figures/images.png figures/sdk.png figures/source-fetching.png
 MANUALS = $(DOC)/$(DOC).html $(DOC)/$(DOC).pdf $(DOC)/eclipse
 FIGURES = figures
 STYLESHEET = $(DOC)/*.css