Improved nothing-to-do-build

- Removed unnecessary dependency of
  "$(my_symlink) :  $(LOCAL_INSTALLED_MODULE)"
  We can generate symlink to nonexistent file.
  Actually in multilib build $(LOCAL_INSTALLED_MODULE) points to file
  that may not be the target file of the symlink and leads to always
  obsolete $(my_symlink) in the above dependnecy.
- Touch by-product in the dummy rule, to make sure the by-product is
  newer than the main-product.

Change-Id: I2f0e0cc197c49f920fa1f6794083b21cdc333c20