support variable-size SPI chip for dummy programmer

This is designed for firmware updater to pack firmware image
preserving some specific partitions in any size.

BUG=none
TEST=ran the command line below:
$ flashrom -p dummy:image=${TMP_FILE},size=16777216, \
emulate=VARIABLE_SIZE -w ${IMG} -V -f

$ flashrom -p dummy:image=${TMP_FILE},size=auto, \
emulate=VARIABLE_SIZE -w ${IMG} -V -f

Signed-off-by: Namyoon Woo <namyoon@google.com>
Change-Id: Iff266e151459561b126ecfd1c47420b385be1db2
Reviewed-on: https://review.coreboot.org/c/flashrom/+/44879
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
3 files changed