google-usb-dyamic: support setting host MAC from BMC This adds a MacIdx field from EM that indexing the MAC list reversely, and set it as the host MAC. Tested: On a machine the host MAC address is correctly set. Fusion-Link: https://fusion2.corp.google.com/10b38e56-a8f0-3b09-8e81-5f2a3654fc4 Google-Bug-Id: 392923910 Change-Id: I8eaba4b32a7fc280d2151b1120e458f8e160b874 Signed-off-by: Yuxiao Zhang <yuxiaozhang@google.com>