nebraska: Fix a few issues

- A nit: Eliminate all \ line breakers.

- Set the metadata signature to empty string when its value is null
  otherwise the XML element tree will break.

- Attach / at the end of update and install payload addresses otherwise
  the update engine just concatenates the base address to the payload
  file name and if there is no /, the address will be invalid.

- sha256_hex is in base64 format (generated by paygen), not hex. convert
  it when reading from json files.

TEST=cros flash

Change-Id: Ie9dcaeb35be6c02d9387d1c9a25fb9fda1fed554
Commit-Ready: Amin Hassani <>
Tested-by: Amin Hassani <>
Legacy-Commit-Queue: Commit Bot <>
Reviewed-by: Xiaochu Liu <>
3 files changed