remeber the base product NVRA (bnc#873885)

(instead of the zypp product reference which might be invalidated)
to properly create the /etc/products.d/baseproduct symlink
changed FindBaseProduct() to RememberBaseProduct()

to share some code
changes

- 3.1.9
review fixes - added comments, indenting fix
Merge pull request #22 from yast/base_product_NVRA

Remeber the base product NVRA (name, version, release, arch)