runqemu: respect IMAGE_LINK_NAME

runqemu: respect IMAGE_LINK_NAME

* when searching for qemuboot.conf
* don't assume that IMAGE_LINK_NAME is always
  <rootfs>-<machine> (with <rootfs>-<machine>.qemuboot.conf)

* runqemu: use IMAGE_LINK_NAME set by testimage.bbclass or query with bitbake -e

* testimage.bbclass was setting DEPLOY_DIR which I don't see used
  anywhere else, so I assume it was supposed to be DEPLOY_DIR_IMAGE as mentioned
  in corresponding runqemu code, do the same with IMAGE_LINK_NAME variable

* add virtual/kernel as bitbake -e target in run_bitbake_env to make
  sure IMAGE_LINK_NAME is defined (kernel-artifact-names.bbclass inherits
  image-artifact-names.bbclass as well)

* improve .qemuboot.conf search
  1st search for file matching the rootfs and only when not found
  try again with .rootfs suffix removed

[YOCTO #12937]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 716eb55bb963db7b02d985849cb025898aabc855)
Signed-off-by: Steve Sakoman <steve@sakoman.com>