build: create JSON files containing image info

build: create JSON files containing image info

The JSON info files contain details about the created firmware images
per device and are stored next to the created images.

The JSON files are stored as "$(IMAGE_PREFIX).json" and contain some
device/image meta data as well as a list of created firmware images.

An example of openwrt-ramips-rt305x-aztech_hw550-3g.json

    {
      "id": "aztech_hw550-3g",
      "image_prefix": "openwrt-ramips-rt305x-aztech_hw550-3g",
      "images": [
        {
          "name": "openwrt-ramips-rt305x-aztech_hw550-3g-squashfs-sysupgrade.bin",
          "sha256": "db2b34b0ec4a83d9bf612cf66fab0dc3722b191cb9bedf111e5627a4298baf20",
          "type": "sysupgrade"
        }
      ],
      "metadata_version": 1,
      "supported_devices": [
        "aztech,hw550-3g",
        "hw550-3g"
      ],
      "target": "ramips/rt305x",
      "titles": [
        {
          "model": "HW550-3G",
          "vendor": "Aztech"
        },
        {
          "model": "ALL0239-3G",
          "vendor": "Allnet"
        }
      ],
      "version_commit": "r10920+123-0cc87b3bac",
      "version_number": "SNAPSHOT"
    }

Signed-off-by: Paul Spooren <mail@aparcar.org>
(backported from commit 881ed09ee6e23f6c224184bb7493253c4624fb9f)
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>