runners: jlink: add support for J-Link tunnels

This adds support for J-Link tunnels, which run on top of an IP network
and therefore uses the -IP option. J-Link tunnels are identified by a
tunnel: prefix instead of a bare IP address. This change checks for the
presence of such a prefix, and choses the -IP transport option if the
tunnel prefix is found.

This has been tested with J-Link Remote Server v7.98g and the SEGGER
tunnel option.

Signed-off-by: Adam Dunkels <adam@dunkels.com>