Add support for reading the current flash contents from a file

When developing software that has to be flashed to a flash chip to be
executed, it often takes a long time to read the current flash contents
(for flashrom to know what pages to erase and reprogram) each time
when writing the new image. However, when the flash was just reprogrammed,
its current state is known to be the previous image that was flashed
(assuming it was verified).

Thus, it makes sense to provide that image as a file for the flash contents
instead of wasting valuable time read the whole flash each time.

Original patch has been created by Paul Kocialkowski, the previous version:
http://patchwork.coreboot.org/patch/4414/
Now it has been ported to the latest source code of flashrom master branch

Change-Id: Idf153b6955f37779ae9bfb228a434ed10c304947
Signed-off-by: Mike Banon <mikebdp2@gmail.com>
Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
---
M cli_classic.c
M flash.h
M flashrom.c
M libflashrom.h
4 files changed, 65 insertions(+), 27 deletions(-)