olpc-update server selection

[Daniel Drake]

Hi Martin,

Going back to recent olpc-update changes...
The current logic of choosing which server(s) to query is not really
suitable for places where the OATS server is not the school server, but
there are school servers online too.

In these situations there will be a responsive OATS server on the
schoolserver, but the one we want them to contact is at a different
address. With the current logic, the fact that the school server
responds means that the other one is not try.

I propose we change the logic to this:

- If a server can be read from ANTITHEFT_SERVER_FILE then use it.
  (this file will not be shipped in our builds, but deployments can add 
   it)
- Otherwise default to http://schoolserver

Then the out-of-the-box configuration will still talk to the school
server, and deployments can override that by creating the
ANTITHEFT_SERVER_FILE when necessary.

antitheft.laptop.org drops out of the picture, but that is not used by
any deployment, and if we did want to use it for providing updates info
to any possible future G1G1 customers then we could just ship the
relevant ANTITHEFT_SERVER_FILE in a G1G1 build.

Thoughts?
Daniel