iPXE will detect timeout failures in several situations: network
link-up, DHCP, TCP connection attempts, unacknowledged TCP data, etc.
This does not cover all possible circumstances.  For example, if a
connection to a web server is successfully established and the web
server acknowledges the HTTP request but never sends any data in
response, then no timeout will be triggered.  There is no timeout
defined within the HTTP specifications, and the underlying TCP
connection will not generate a timeout since it has no way to know
that the HTTP layer is expecting to receive data from the server.

Add a "--timeout" parameter to "imgfetch", "chain", etc.  If no
progress is made (i.e. no data is downloaded) within the timeout
period, then the download will be aborted.

Signed-off-by: Michael Brown <mcb30@ipxe.org>