testexport: fallback for empty IMAGE_LINK_NAME

if IMAGE_LINK_NAME is set empty to disable the symlinking
for image artifacts in deploy, testexport fails, as the path assembly
is incorrect.
In that case fallback to IMAGE_NAME

(From OE-Core rev: 0c1d098e6dd08fa3a5aafca656457ac6badcef89)

(From OE-Core rev: a821a5810ae6edba5882a43e3c60c142f2acda9d)

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>