[PATCH 0/2] Add support for containerized SDKs

[Silvano Cirujano Cuesta <silvano.cirujano-cuesta@siemens.com>]

This patch series extends the SDK creation task `populate_sdk` to enable
the creation of container images (different formats that can be
simultaneously generated supported) providing the SDK. Containerized
SDKs are easy to distribute and run and are therefore quickly spreading.

The SDK format available until this patch (.tar.xz of SDK rootfs that
can be chrooted to) remains available and is the default. Even for those
wanting to use the here called `tar-xz` format it's possible to use
container images as "packaging" medium (simple to get tools -script,
skopeo,...- can help on this, if needed).

More information about its usage is documented in the file
docs/user_manual.md.

A PoC/demo of this functionality has been created based on the project
https://github.com/siemens/meta-iot2050.
Jan Kiszka already tested and liked it! =>
https://github.com/siemens/meta-iot2050/issues/86#issuecomment-768907845

In order to get a feeling about its usage (you need Docker or Podman),
follow these simple copy&paste instructions:
https://github.com/Silvanoc/meta-iot2050/blob/master/kas/BUILDING-SDK-CONTAINER.md#running-the-sdk
Build instructions are available in the upper part of that document.

Two new dependencies are required to create containerized SDKs (as
specified in the documentation).

Typical container image management actions (e.g. push an image to a
container image regitry) are out of scope. Available tools (Docker,
Skopeo, Buildah, Podman,...) should be used for these actions.

This patch series addresses the comments and concerns raised in the RFC
called "support generation of sdk container images".

A patch will follow this one to get the dependencies into the container
images being provided by the project
https://github.com/siemens/kas (for `kas-container`, for example).

Silvano Cirujano Cuesta (2):
  sdk: support creation of container image
  docs: document usage of sdk container images

 doc/user_manual.md                       |  79 +++++++++++++++++
 meta/classes/image-sdk-extension.bbclass | 104 +++++++++++++++++++++--
 2 files changed, 176 insertions(+), 7 deletions(-)

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2.30.0