improve_kernel_cve_report: add option to read debugsources.zstd

improve_kernel_cve_report: add option to read debugsources.zstd

Adding option to be able to import debugsources.zstd directly.
The linux-yocto-debugsources.zstd is generated in every build and
does not require any additional configuration.

In contrast, SPDX_INCLUDE_COMPILED_SOURCES needs to be explicitly
added and increases build time.

Signed-off-by: Daniel Turull <daniel.turull@ericsson.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>