image-artifact-names: add IMAGE_MACHINE_SUFFIX variable

image-artifact-names: add IMAGE_MACHINE_SUFFIX variable

* to make it easier for projects to avoid default -${MACHINE} suffix if
  the ${MACHINE} named DEPLOY_DIR_IMAGE works better for them

* also use IMAGE_LINK_NAME in IMAGE_NAME to make it more clear
  that IMAGE_NAME is the same as IMAGE_LINK_NAME but with version
  suffix

* adding it as separate variable helps us to catch the cases
  where we didn't respect ${IMAGE_LINK_NAME} variable and just used
  the common default ${IMAGE_BASENAME}-${MACHINE}.

[YOCTO #12937]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>