download/cvs: add a 10 minute timeout

download/cvs: add a 10 minute timeout

Apparently, CVS servers can be deadlocked and in that case clients will
retry connecting to them indefinitely. Cfr.
http://autobuild.buildroot.net/results/23d/23d1034b33d0354de15de2ec4a8ccd0603e8db78/build-end.log
Apparently, the sf.net CVS server got in such a deadlock on 2018-05-18,
and almost 2 weeks later it is still not fixed.

Instead of just hanging, we should fall back on BR2_SECONDARY_SITE. To
achieve this, it's sufficient to add a timeout to the CVS command.

The timeout value is of course arbitrary. However, we can assume that
nobody will be putting large projects under CVS any more. So if the
download takes more than 5 minutes, it's probably broken. Let's put the
timeout at 10 minutes then.

Fixes:
http://autobuild.buildroot.net/results/db3/db33d4fa507fb3b4132423cd0a7e25a1fe6e4105
http://autobuild.buildroot.net/results/b6d/b6d927dcc73ac8d754422577dacefff4ff918a5c
http://autobuild.buildroot.net/results/23d/23d1034b33d0354de15de2ec4a8ccd0603e8db78
http://autobuild.buildroot.net/results/127/1272a3aa3077e434c9805ec3034f35e6fcc330d4

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>