scripts: add possibility to not override default connection timeout

Allow '-1' to be specified as CONNECTION_TIMEOUT to disable the use
of the connection timeout for wget.

Signed-off-by: Cody P Schafer <dev@codyps.com>
Message-Id: <cb33f8c2cbaf802d4f04.1399687632@localhost>
Patchwork-Id: 347582

 reporter_name="Ioannis E. VENETIS"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

 reporter_comment=""