tools: add firmware recovery helpers

tools: add firmware recovery helpers

This adds a set of helper functions with the aim to make it possible
to flash mass-market devices without RTFMing altogether (i.e. to
obsolete GPL-violating proprietary tjtag and other similar software).

Real-life tested on an RT-N16 and WRT54GL.

Change-Id: I197a9b28a5f386803f081057c4b4ebf2f9c447b1
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1850
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>