futility: Add read command

Add a command that reads AP firmware to a specified file path.

BUG=b:260531154
BRANCH=None
TEST=FEATURES=test emerge-grunt vboot_reference
TEST=futility read /tmp/bios
TEST=futility read /tmp/bios -p ec
TEST=env SERVOD_NAME=grunt futility read /tmp/bios --servo

Change-Id: I82fe0381b6f61ca4d67a9f5c27353e18ed4abe39
Signed-off-by: Evan Benn <evanbenn@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/4075310
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Commit-Queue: Edward O'Callaghan <quasisec@chromium.org>