bitbake: user-manual: Added new chapter to cover BB execution.

bitbake: user-manual: Added new chapter to cover BB execution.

This commit added need files, cleaned up bogus links, and
adjusted the build file, which is user-manual.xml.

The new chapter file is "user-manual-execution.xml". The file
has base text from the bitbake section from the "Closer Look"
chapter of the YP ref-manual.  This means that I had to initially
include some figures from that book for now.  I am not including
them as part of this commit though as they will likely get renamed.
I made sure that all links were removed that were specific to the
YP ref-manual.

Another change was in the user-manual.xml file to include
the new chapter as part of the build.

Another block of text for the new chapter was the "Running a Task"
section from the user-manual-metadata.xml file.  Consequently, that
section was deleted from the "Metafile" chapter.

Another block of text for the new chapter was the "Parsing
and Execution" section from the user-manual-metadata.xml file.
Consequently, that section was deleted from the "Metafile chapter.

(Bitbake rev: f3b21d1fb711f9625d2ac92d4f4fe0f269242bd7)

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