VBMETA data for another partition

The vbmeta() template in //build/images/vbmeta.gni allows for the addition of extra, arbitrary, hash descriptors in the vbmeta image that's generated for a zbi. The need for these are product-specific.

The format of these are:

{
    "type": "partition",
    "name": "partition_name",
    "size": "104448",
    "flags": "1",
    "min_avb_version": "1.1"
}

• name = (string) The name of the partition as it to appear in the vbmeta image
• size = (int as string) The size of the partition in bytes
• flags = (int) The bit-wise set of flag values
• min_avb_version = (version x.y as a string) the minimum avb version that the resultant vbmeta image requires if it includes this descriptor.

Note: These files cannot contain any comments, and must strictly conform to the JSON spec.