ref-manual: Added KERNEL_ARTIFACT_NAME and adjusted referencing variables.

ref-manual: Added KERNEL_ARTIFACT_NAME and adjusted referencing variables.

The KERNEL_ARTIFACT_NAME variable is used throughout to set the names
of build artifacts.  Rather than repeat informaiton about
KERNEL_ARTIFACT_NAME in the many variables that use it, I added
a new entry for the variable.  This also impacted the descriptions
of the variables that were repeating information.  I updated those
variable descriptions as well.

(From yocto-docs rev: 5750d30be21e17fe21d89254925dc0e773c08e66)

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