tcl: add flash programming helper

This adds a program proc that simplifies using OpenOCD as a standalone
programmer.

Change-Id: I6ece492cd878c170b734e8bb2e09fe8c4557d5a6
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1199
Tested-by: jenkins
Reviewed-by: Jörg Fischer <turboj@gmx.de>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>