meta-phosphor: phosphor-software-manager: Add symlink as git

Add a symbolic link to phosphor-software-manager.bb named _git.bb and
set the preferred version for the recipe.

(From meta-phosphor rev: 25045bc375c9ef7cd8fdf0a9adfab406384d4c8c)

Change-Id: Ia1441aedb774c94dc1b32bac21e8800ae9654cda
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>