selftest: multiconfig-image-packager: try to respect IMAGE_LINK_NAME

* this still assumes that IMAGE_LINK_NAME will contain IMAGE_BASENAME
  which will be BPN 'multiconfig-image-packager' and that replacing
  it with 'core-image-minimal' will match with the actual IMAGE_LINK_NAME
  from core-image-minimal recipe - there is no good way to query
  core-image-minimal's context, but this is still closer than assuming:
  core-image-minimal-${MCMACHINE}.${MCIMGTYPE}
  which works only with the current default:
  IMAGE_LINK_NAME ?= "${IMAGE_BASENAME}-${MACHINE}"

[YOCTO #12937]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>