Flatcar: add target for Flatcar-compatible systemd sysext images

[kartikjoshi21]

This PR adds an initial Flatcar build target to Dalec focused on producing Flatcar-compatible systemd system extension images (.raw).

• Adds flatcar/testing/sysext which builds a .raw sysext using the Flatcar SDK image as the sysext worker.
• Introduces an optional SysextEnv(...) hook so a target can supply default DALEC_SYSEXT_* values for build_sysext.sh.DALEC_SYSEXT_* build args always override defaults.
• If DALEC_SYSEXT_OS_VERSION_ID is set and DALEC_SYSEXT_SYSEXT_LEVEL is not explicitly set, the default SYSEXT_LEVEL is dropped to avoid over-constraining extension matching.

Default behavior for Flatcar sysext
The Flatcar target provides defaults aligned with Flatcar expectations:

• DALEC_SYSEXT_OS_ID=flatcar
• default output image name is the spec name (produces .raw)
• default DALEC_SYSEXT_SYSEXT_LEVEL=1.0 (unless OS_VERSION_ID is pinned without an explicit level)

Tests
Adds unit coverage for:

• filtering of DALEC_SYSEXT_* build args
• merging target defaults with build-arg overrides
• the OS_VERSION_ID vs SYSEXT_LEVEL precedence rule

Fixes: #969

[Copilot]

Pull request overview

This PR adds a new Flatcar build target to Dalec, enabling production of Flatcar-compatible systemd system extension (.raw) images. The implementation follows established patterns in the codebase while introducing an optional interface for distro-specific sysext configuration.

Changes:

• Adds new Flatcar target with sysext support using the Flatcar SDK for building
• Introduces optional sysextEnvProvider interface allowing targets to supply default DALEC_SYSEXT_* environment variables
• Implements smart merging logic that drops default SYSEXT_LEVEL when OS_VERSION_ID is pinned to avoid over-constraining extension matching

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
targets/linux/flatcar/flatcar.go	New Flatcar target implementation using composition pattern, delegating to Ubuntu Noble for base functionality while providing Flatcar-specific SDK worker and sysext defaults
targets/linux/distro_handler.go	Adds optional sysextEnvProvider interface and environment merging logic to pass DALEC_SYSEXT_* build args to build_sysext.sh with proper default handling
targets/linux/build_sysext.sh	Enhances script to support configurable output naming, OS ID matching, VERSION_ID/SYSEXT_LEVEL parameters, and creates proper symlinks for Flatcar naming conventions
targets/linux/sysext_env_test.go	Unit tests for DALEC_SYSEXT_* environment filtering and merging logic including the OS_VERSION_ID/SYSEXT_LEVEL precedence rule
targets/plugin/init.go	Registers new Flatcar target in plugin initialization
load.go	Allows DALEC_SYSEXT_* build args without requiring spec declaration

[Copilot]

The indentation here uses spaces instead of tabs, which is inconsistent with the rest of the file that uses tabs for indentation. Please change the leading spaces to a tab to maintain consistency.

Suggested change

	ln -sf "${BASE_REL}" "${REL_DIR}/extension-release.${IMAGE_NAME}"
	ln -sf "${BASE_REL}" "${REL_DIR}/extension-release.${IMAGE_NAME}"