layout: Add -i <region>[:<file>] support

Add an optional sub-parameter to the -i parameter to allow building the
image to be written from multiple files. This will also allow regions to
be read from flash and written to separate image files.

This is a rebase of a patch that was ported from chromiumos. A lot of
things have changed, but the idea is the same.

Original patch by Louis Yung-Chieh Lo <yjlou@chromium.org>:
Summary: Support -i partition:file feature for both read and write.
Commit: 9c7525f
Review URL: http://codereview.chromium.org/6611015

Ported version by Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
and Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>:
Summary: [PATCH 2/6] layout: Add -i <region>[:<file>] support.
Review URL: https://mail.coreboot.org/pipermail/flashrom/2013-October/011729.html

Change-Id: Ic5465659605d8431d931053967b40290195cfd99
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Co-Authored-by: Edward O'Callaghan <quasisec@google.com>
---
M cli_classic.c
M dummyflasher.c
M flash.h
M flashrom.8.tmpl
M flashrom.c
M layout.c
M layout.h
7 files changed, 233 insertions(+), 21 deletions(-)