swupdate-common.bbclass: Handle empty IMAGE_LINK_NAME
swupdate-common.bbclass: Handle empty IMAGE_LINK_NAME

When IMAGE_LINK_NAME is empty or when IMAGE_LINK_NAME is equal to
IMAGE_NAME, this results in an error like:
  Exception: FileExistsError: [Errno 17] File exists:
  'my-image-swu-sd-tdpzu9.swu' -> 'my-image-swu-sd-tdpzu9.swu'

To solve this, check if IMAGE_LINK_NAME is valid before attempting to
create the symlink.

Signed-off-by: Mike Looijmans <a3a80a5bf09f2a16fb39154c4e7d0b2d849b0dfb@topic.nl>
Signed-off-by: Stefano Babic <2c2905e0c2877c7e4ce40c7425e972302953fda5@denx.de>

sbabic/meta-swupdate,sbabic/meta-swupdate