This patch adds support for a new local.conf variable called
BB_TRUSTED_NETWORK.  BB_TRUSTED_NETWORK holds a list of hostnames that the user
trusts as a source for downloading content.  If network access is enabled and
the user has configured trusted hosts, then any hosts that are not in the list
will cause an error to occur at fetch.  Any mirrors and pre-mirrors that are
not in the list will result in warnings that these locations will not be used.

The BB_NO_NETWORK variable still stops all network access.

Please see the comments in the patch for more details and example usage.