crossystem: change mosys calls to use the subprocess library

The new subprocess_run library function provides a unit-tested way to
call subprocesses and collect output.

BUG=chromium:1030473
BRANCH=none
TEST=On drallion and druwl, crossystem works for both reading and writing

Change-Id: I072d91cf68ee91c663652d2c0433ef8bbedad7d7
Signed-off-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1955806
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
2 files changed