cve-check-tool: Fixes for recipe sysroots

cve-check-tool: Fixes for recipe sysroots

* Use --enable-relative-plugins so cve-check-tool looks for
  loadable modules relative to binary location instead of
  hard-coding a wrong sysroot location
* do_populate_cve_db() assumes that the binary cve-check-update is in
  the sysroot. Ensure that this is true by adding a task dependency

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>