improve_kernel_cve_report: do not use custom version

improve_kernel_cve_report: do not use custom version

When using the version specified in cve-summary.json, we need to
remove the suffix containing the custom version to match the
versions from the CVEs.

This patch truncates the version from cve-summary.json to use only
the base version of the kernel.

This is only applicable for kernels where the user has added their
own version.

Signed-off-by: Daniel Turull <daniel.turull@ericsson.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>