bitbake-user-manual: Added information for using single quotes

bitbake-user-manual: Added information for using single quotes

Fixes [YOCTO #10293]

In the section about setting variables, I added a paragraph that
explains the use of single quotes when setting a variable.  The
case covers when you must have the double quote charater as part
of your variable's value.

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>