emmc-image: support vendor workflow Vendor does not have payload library and they use in-place update and don't care which half. Tested: flag is correctly set and script picks half 0 Fusion-Link: fusion2 N/A cannot tested by presubmit Google-Bug-Id: 359666294 Change-Id: I67683a85e19b43e0bdbf57ab95d8cb757fca9a18 Signed-off-by: yuxiaozhang <yuxiaozhang@google.com>