[OE-core] GIT_CORE_CONFIG needed? set by default?

[McClintock Matthew-B29882]

All,

I was having more issues with git over our proxy (again) and realized
we were not setting GIT_CORE_CONFIG. This enables creation of the
gitconfig file - otherwise it's not created. I'm not entirely sure if
it falls back to the users gitconfig or not - point is I was thinking
we should have this set by default so the gitconfig is created for the
git related fetching and we can count on a consistent environment when
it comes to debugging (we don't want to get messages with git failures
only to find out the user has some weird foo in their own gitconfig).

Comments?

-M