populate_sdk_ext: keep SDK_TARGETS so SPDX/SBOM tasks remain in locked sigs

populate_sdk_ext: keep SDK_TARGETS so SPDX/SBOM tasks remain in locked sigs

Fixes [YOCTO #15853]

The filtering in populate_sdk_ext.bbclass was removing all image targets from
the locked signatures, including the main SDK images (SDK_TARGETS). As a
result, their tasks including the image SPDX/SBOM tasks were not added to
locked-sigs.inc.

Without these entries, the eSDK lacked the necessary sstate, and the installer
attempted to run the missing tasks during installation, leading to unexpected
task execution errors.

This patch keeps SDK_TARGETS (and their multilib variants) in the locked
signatures so their SPDX/SBOM tasks are included. With those tasks in the
sstate cache, the eSDK installs cleanly without needing to re-run anything.

Signed-off-by: Jayasurya Maganuru <Maganuru.Jayasurya@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>