spdx30_tasks: Add concluded license support with SPDX_CONCLUDED_LICENSE

spdx30_tasks: Add concluded license support with SPDX_CONCLUDED_LICENSE

Add hasConcludedLicense relationship to SBOM packages with support for
manual license conclusion override via SPDX_CONCLUDED_LICENSE variable.

The concluded license represents the license determination after manual
or external license analysis. This should be set manually in recipes or
layers when:

1. Manual license review identifies differences from the declared LICENSE
2. External license scanning tools detect additional license information
3. Legal review concludes a different license applies

The hasConcludedLicense relationship is ONLY added to the SBOM when
SPDX_CONCLUDED_LICENSE is explicitly set. When unset or empty, no
concluded license is included in the SBOM, correctly indicating that
no license analysis was performed (per SPDX semantics).

When differences from the declared LICENSE are found, users should:

1. Preferably: Correct the LICENSE field in the recipe and contribute
   the fix upstream to OpenEmbedded
2. Alternatively: Set SPDX_CONCLUDED_LICENSE locally in your layer when
   upstream contribution is not immediately possible or when the license
   conclusion is environment-specific

The implementation checks both package-specific overrides
(SPDX_CONCLUDED_LICENSE:${PN}) and the global variable, allowing
per-package license conclusions when needed.

The concluded license expression is automatically de-duplicated by
add_license_expression() to avoid redundant license objects in the SBOM.

The variable is initialized in spdx-common.bbclass with comprehensive
documentation explaining its purpose, usage guidelines, and examples.

Example usage in recipe or layer:
  SPDX_CONCLUDED_LICENSE = "MIT & Apache-2.0"
  SPDX_CONCLUDED_LICENSE:${PN} = "MIT & Apache-2.0"

Signed-off-by: Stefano Tondo <stefano.tondo.ext@siemens.com>
Reviewed-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>