archiver.bbclass: Reduce some duplication for function get_licenses
The content to modify this bbclass is as follow:
- Use the existing functions to get license as a directory instead of
rewriting it for avoiding code duplication.
- Use SPDXLICENSEMAP to map licenses
[YOCTO #2473]
Signed-off-by: Xiaofeng Yan <xiaofeng.yan@windriver.com> Signed-off-by: Elizabeth Flanagan <elizabeth.flanagan@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
