ANDROID: add script to fetch android kernel config fragments

The Android kernel config fragments now live in a separate repository.
To prevent others from having to search for this location, add a script
to fetch and unpack the fragments.

Update .gitignore to include these fragments.

Change-Id: If2d4a59b86e4573b0a9b3190025dfe4191870b46
Signed-off-by: Steve Muckle <smuckle@google.com>