util/flash_ec: add --dry_run flag
util/flash_ec: add --dry_run flag

I've found it useful discover what commands are being run to actually
flash the EC, without actually flashing it.

This CL adds a new flag, --dry_run, which prints the commands
necessary to reproduce the flash, instead of running them.  Commands
which probe hardware presence (e.g., dut_control_get) are not printed.

BUG=none
BRANCH=none
TEST=manual, observe operation with scarlet connected, both with and
     without --dry_run

Change-Id: Ib7de66176625878370d5b24a5fb7a32b37965cb8
Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2226505
Reviewed-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec