Adds 'image_url' JSON parameter to an SDK entry

Adds 'image_url' JSON parameter to an SDK entry

Adds a link to installable target image corresponding to the current
SDK. The easest way to implement this was to manage a pair of suffix
for earch source of SDKs (AGL or IoT.bzh), The input data are
refactored as following: each source has an URL prefix, a list of
directories (possibly including wildcard) to be browsed, relative to
prefix URL, and a pair of suffixes : one for SDK, one for ilage
(relative to prefix + directory).

When an SDK installation file is found, the corresponding image is
searched by replacing the SDK suffix by the image one, requesting this
URL, and grepping for a filename matching agl-demo-plaform-crosssdk-*
With different possible extensions:
- .vmdk(.xz) in case of qEMU image, or
- .rootfs.wic(.xz), when avaible, or
- .rrotfs.tar(.xz)

If no matching image file is found, the image_url is set to the
directory URL containing the image file candidates.

If requesting the image directory URL content leads to server error
(mainly error 404 Not Found, but also any other error), the image_url
value will be set to null in the JSON output.

Makes the trace system more flexible, using an verbose -v|vv option.

KNOWN ISSUES:
    - '-nogfx' images are not named agl-demo-plaform-crosssdk-* on the
      AGL server. image_url is set to the image directory URL for such
      SDKs/images
    - on IoT.bzh, older images directories are named 'image' while
      newer ones are named 'images'. Advantage given to newer images,
      this lead to have older SDKs' image_url set to null.

Change-Id: I8c029d3ebbdef1a096c5cb225fd40405e91b312d
Signed-off-by: Johann CAHIER <johann.cahier@iot.bzh>