fetch2: Add BB_ALLOWED_NETWORKS support

fetch2: Add BB_ALLOWED_NETWORKS support

BB_ALLOWED_NETWORKS is a list of hosts that the fetcher will be allowed
to use when BB_NO_NETWORK is not set.

If BB_NO_NETWORK is set, then networking is still disabled.

If BB_ALLOWED_NETWORKS is not set, the behaviour remains the same as
today.

If BB_NO_NETWORK is NOT set, and BB_ALLOWED_NETWORKS is configured, then
only the hosts in the list are usable by the fetcher.

eg:
BB_ALLOWED_NETWORKS="yoctoproject.org git.gnu.org"
The fetcher will be able to download from yoctoproject.org, git.gnu.org,
but not ftp.gnu.org or any other hostname that is not in the list.

There is also limited support for wildcards on the beginning of the
hosts, so BB_ALLOWED_NETWORKS="*.gnu.org" with match git.gnu.org and
ftp.gnu.org as well as foo.git.gnu.org

Signed-off-by: Liam R. Howlett <Liam.Howlett@WindRiver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>