image.py: Allow IMAGE_LINK_NAME to be empty

image.py: Allow IMAGE_LINK_NAME to be empty

When IMAGE_LINK_NAME is empty, OE will try to create a "blank" link instead
of just skipping it. The code checks for "link_name is not None" which will
never evaluate to true. Change the test to a simple "if link_name:" so it
no longer attempt to create links when the variable is an empty string.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>