With the addition of support for custom opensbi version in commit
5c7166d387 (boot/opensbi: add support for version configuration), we can no
longer be sure that the license file name / hash will be correct in all
cases, so only specify COPYING.BSD when _LATEST_VERSION is used, similar to
how we do it for the Linux kernel.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>