transport: make 'transport select' auto-select the first available transport if not set

This should allow most of the existing configurations for older
versions to remain compatible without forcing the user to change his
or her config to explicitly select transport.

Also in some circumstances can remove the need to chain a "-c transport
select X" when building custom configs on the command line, which seems
like a common new user pitfall.

Change-Id: Ic87a38c0b9b88e88fb6d106385efce2f39381d3d
Suggested-by: Petteri Aimonen <jpa@git.mail.kapsi.fi>
Signed-off-by: Angus Gratton <gus@projectgus.com>
Reviewed-on: http://openocd.zylin.com/2551


Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>