make args for -r/-w/-v non-positional and optional

Ported from chromiumos:
https://chromium-review.googlesource.com/#/c/60515/

This makes a filename following -r, -w, and -v operations non-
positional. The first argument that does not begin with a hyphen (-)
is the filename for -r/-w/-v. If no such argument exists, then
-r/-w/-v options apply to files specified for individually included
regions via -i.

This has a few side-effects:
1. It allows us to omit the ROM-sized filename entirely when a
   filename is provided for a particular region when using -i. For
   example, "flashrom -i FOO:foo.bin -r".

2. It allows, for better or worse, the filename be specified anywhere
   on the command line after the program name. Our scripts and docs
   should still specify the file after -r/-w/-v for clarity.

For our purposes, if a filename is given for every included region
(-i region:filename) then the user does not need to specify a filename
after -r/-w/-v. However, if any -i option does not specify a filename,
then a file must be specified after -r/-w/-v.

The syntax will be backwards compatible for now so that one can still
mix -i options with and without the added filename specifier for each
region.

BUG=chromium:263495
BRANCH=none
TEST=manual (see notes below)

1. Write random data to RW_UNUSED region (on snow in this case)
   without requiring an argument to -w. See that only RW_UNUSED
   is erased and written, and that verify works:
   dd if=/dev/urandom of=rw_unused.bin bs=1k count=1 conv=notrunc
   flashrom -p host -i RW_UNUSED:rw_unused.bin -w -V
   flashrom -p host -i RW_UNUSED:rw_unused.bin -v -V

2. Same as above, but specify a dummy file to test syntax
   backwards compatibility:
   dd if=/dev/urandom of=rw_unused.bin bs=1k count=1 conv=notrunc
   dd if=/dev/urandom of=random_4M.bin bs=1M count=4
   flashrom -p host -i RW_UNUSED:rw_unused.bin -w random_4M.bin -V
   flashrom -p host -i RW_UNUSED:rw_unused.bin -v random_4M.bin -V

3. Test that dumping RW_UNUSED and GBB regions without -r arg dumps
   two files and that they can be used to verify the content:
   flashrom -p host -i RW_UNUSED:rw_unused.bin -i GBB:gbb.bin -r
   flashrom -p host -i RW_UNUSED:rw_unused.bin -i GBB:gbb.bin -v

4. Same as above, but with dummy file:
   flashrom -p host -i RW_UNUSED:rw_unused.bin -i GBB:gbb.bin -r x.bin
   flashrom -p host -i RW_UNUSED:rw_unused.bin -i GBB:gbb.bin -v x.bin

Change-Id: Iefbcb7dc4fefe26f5afd1292dfd5c1687fa62803
Reviewed-on: https://gerrit.chromium.org/gerrit/60515
Reviewed-by: Yung-Chieh Lo <yjlou@chromium.org>
Commit-Queue: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
---
M cli_classic.c
M flashrom.c
M layout.c
M layout.h
4 files changed, 210 insertions(+), 22 deletions(-)