Makefile: Added publish tag so the book can be published.

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
user-manual-metadata.xml: Sections to handle variable setting added.

Fixes [YOCTO #5507]

I provided further explanation in the "Basic Variable Setting"
section to note that trailing and leading blanks are not stripped
from a value when set (e.g. VARIABLE = " value").

I added a new section "Null and Blank Setting" explaning that setting
a variable to "" and " " are two different things.

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
user-manual-metadata.xml: Edits to "Basic Variable Setting"

Fixes [YOCTO #5507]

I applied some edits from Richard's review to the fix for
handling variable setting.  Moved the new section I created into
the existing "Basic Variable Setting" section.

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
user-manual-metadata.xml: Added varflag for vardepvalueexclude.

Fixes [YOCTO #5897]

Added several new varflags to the existing list in the
"Variable Flags" section.  The key one being the
"vardepvalueexclude" flag.

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>