Integrate gitlab-ci/add-build-version-job and 7 more

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@cdd47023

Introduce separate variables to set RELEASE_NAME and RELEASE_VERSION

The expressions to calculate RELEASE_NAME and RELEASE_VERSION cannot be
stored within these variables themseselves. If set on the trigger level,
they would override the calculated values in build-version.env then,
as trigger variables always have the highest precedence.

Use separate variables RELEASE_VERSION_EXPRESSION and
RELEASE_NAME_EXPRESSION to define how RELEASE_VERSION and RELEASE_NAME
are calculated instead.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@372d192b

Use RELEASE_VERSION and RELEASE_NAME from build-version job

Instead of passing these variables from stage to stage or regenerating
their values in later stages, use the ones set in the build-version job
at all places.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@cad30c20

Remove RELEASE_SUFFIX variable

The RELEASE_NAME variable can be set directly now, so there's no need to
have a dedicated RELEASE_SUFFIX anymore. A previous configuration like

  RELEASE_SUFFIX: "-custom"

can now be achieved using:

  RELEASE_NAME: "Yocto-${RELEASE_VERSION}-custom"

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@922f49f8

Add yocto version job

This adds a machine-independent job "build-version" which populates the
RELEASE_VERSION and RELEASE_NAME variables, so that following jobs can
use these without depending on the various build jobs.
The variables can be set from the trigger job in a project's
`.gitlab-ci.yml` file. They are eval'ed before saving them to
version.env, so we can use deferred variable expansion or even command
execution to construct their values. This mechanism is already used for
the Flash-N-Go System variables.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@073ae81e

LAVA: rename MACHINE to LAVA_MACHINE

Rename variable in order to avoid confusion with the original MACHINE
variable used everywhere else. Also rename the local Jinja2 variable to
include an underscore to make naming consistent.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@a91baa55

Clean build.env before writing

This had been accidentally removed in
5e36715ef6cf98df4c1b98fedddc0c3c50ed4040.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@1860ca8e

Remove LOG_PREFIX variable

This was used in times when the same code was executed from different
places. We don't do that anymore, so the variable is obsolete.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@3d2dbe4d

Documentation: update job generation chapter