image_types_phosphor: split make_tar_of_images

Move image symlink creation for phosphor-bmc-code-mgmt out
of make_tar_of_images into its own function, so we can run
additional code in between symlink creation and tar file
creation.

Tested: Built witherspoon and palmetto images
Change-Id: I3025db6bb788d7c2bcf5c2a400af647c7c957164
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>