chromium daily test: Switch to ECS.

1. support ECS build on aosp-stable(x86, x86_64).
2. support ECS flash-image on aosp-stable(x86, x86_64).

workaround:
* fastboot not ready so use a prebuilt loader.efi and fastboot.img.
* use specific flash.sh.
* all are at directory of /workspace/server/workaround.