distro-manifest-generator: add support for JSON output format

distro-manifest-generator: add support for JSON output format

Distro build manifest is easier to parse if output is made in a structured
format. Adding JSON output format will allow bindings to read information
more easily.

The files created are now:

On target:
* /etc/platform-info/build (shell format)
* /etc/platform-info/build.json (JSON format)

In image deploy dir (.../tmp/deploy/images/$MACHINE/):
* build-info (shell format)
* build-info.json (JSON format)

In sdk deploy dir (.../tmp/deploy/sdk/):
* poky-agl-<version details>.build-info (shell format)
* poky-agl-<version details>.build-info.json (JSON format)

Bug-AGL: SPEC-720, SPEC-1917

Change-Id: If45d2c5dd96b15ce790aa7f7f97c24f119ad117b
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>