Switch zephyr ec.bin filename to match coreboot

Zephyr EC builders don't actually write the file to zephyrEcName/ec.bin
as assumed, they actually write it to corebootName/ec.bin.

Adjust code to match reality.

BUG=b:356284945
TEST=CQ

Change-Id: Iec05d70f748363ae11573881f0dfaef14fadc053
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/5777461
Auto-Submit: Jeremy Bettis <jbettis@chromium.org>
Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>
Tested-by: Jeremy Bettis <jbettis@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
2 files changed