dummyflasher: Wire variable size feature via opaque infra

Wire "variable size" feature in dummy programmer via opaque
infra. This patch fixes the broken build with CONFIG_DUMMY=no.

Dummyflasher registers opaque master for the case when it is
initialised with EMULATE_VARIABLE_SIZE. Dummy opaque master
emulates read/write/erase as simple memory operations over
`data->flashchip_contents`.

Fixes: https://ticket.coreboot.org/issues/365

TEST=the following scenarious run successfully

Testing build

$ make clean && make CONFIG_DUMMY=no
$ flashrom -h : dummy is not in the list
$ make clean && make CONFIG_EVERYTHING=yes
$ flashrom -h : dummy is in the list

Testing "variable size" feature

$ flashrom -p dummy:size=8388608,emulate=VARIABLE_SIZE -V
$ flashrom -p dummy:size=8388608,emulate=VARIABLE_SIZE -r /tmp/dump.bin -V
$ head -c 8388608 </dev/urandom >/tmp/image.bin
$ flashrom -p dummy:image=/tmp/image.bin,size=8388608,emulate=VARIABLE_SIZE -w /tmp/dump.bin -V

Testing standard flow

$ flashrom -p dummy:emulate=W25Q128FV -V
$ flashrom -p dummy:emulate=W25Q128FV -r /tmp/dump.bin -V
$ head -c 16777216 </dev/urandom >/tmp/image.bin
$ flashrom -p dummy:image=/tmp/image.bin,emulate=W25Q128FV -w /tmp/dump.bin -V

Change-Id: I76402bfdf8b1a75489e4509fec92c9a777d0cf58
Signed-off-by: Anastasia Klimchuk <aklm@chromium.org>
---
M dummyflasher.c
M flashchips.c
M include/chipdrivers.h
3 files changed, 77 insertions(+), 73 deletions(-)