image-artifact-names: introduce new bbclass and move some variables into it

* similar to kernel-artifact-names for other recipes/bbclasses which
  need to use some deployed artifacts

* bitbake.conf: move IMAGE_BASENAME, IMAGE_VERSION_SUFFIX, IMAGE_NAME,
  IMAGE_LINK_NAME variables

* image_types.bbclass: move IMAGE_NAME_SUFFIX variable

* currently IMAGE_NAME_SUFFIX is used only by image.bbclass,
  image_types.bbclass and meta/recipes-core/images/build-appliance-image_15.0.0.bb
  but if it's needed by some recipe which isn't itself an image, then
  it's useful in bitbake.conf, e.g. we have a recipe for creating
  VirtualBox appliances which combines .wic.vmdk with .ovf file to
  create .zip with appliance, but for that we need the filename of
  .wic.vmdk which now contains IMAGE_NAME_SUFFIX
  https://github.com/webOS-ports/meta-webos-ports/blob/4980ce52a43ac6897657602810313af359f0b839/meta-luneos/recipes-core/images/luneos-emulator-appliance.inc#L24

* we were hardcoding .rootfs suffix where needed, but for quite long
  time it's configurable with IMAGE_NAME_SUFFIX since:

  commit 380ee36811939d947024bf78de907e3c071b834f
  Author: Patrick Ohly <patrick.ohly@intel.com>
  Date:   Mon Mar 7 18:07:52 2016 +0100

    image creation: allow overriding .rootfs suffix

  and might not match with hardcoded .rootfs, so make it easier to
  use IMAGE_NAME_SUFFIX where needed even without inheritting whole
  image_types.bbclass

[YOCTO #12937]

(From OE-Core rev: 456b700d51a5052a285a8477304f902c335223be)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/testimage.bbclass

@@ -3,6 +3,8 @@
 # Released under the MIT license (see COPYING.MIT)
 
 inherit metadata_scm
+inherit image-artifact-names
+
 # testimage.bbclass enables testing of qemu images using python unittests.
 # Most of the tests are commands run on target image over ssh.
 # To use it add testimage to global inherit and call your target image with -c testimage