lib/configfragments: add a show-fragments command

lib/configfragments: add a show-fragments command

We can print information on fragments (name, location, description,
etc.), but not their content.

Add a show-fragment command to do that. It can be used as follows:

  $ bitbake-config-build show-fragment core/yocto/sstate-mirror-cdn

And prints:

  .../meta/conf/fragments/yocto/sstate-mirror-cdn.conf:

  BB_CONF_FRAGMENT_SUMMARY = "Use prebuilt sstate artifacts for standard Yocto build configurations."
  BB_CONF_FRAGMENT_DESCRIPTION = "The Yocto Project has prebuilt artefacts available for standard build configurations. \
  ...

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>