boot/opensbi: only check/reference COPYING.BSD when _LATEST_VERSION is used

With the addition of support for custom opensbi version in commit
5c7166d387b (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>

boot/opensbi/opensbi.mk

@@ -20,7 +20,9 @@ OPENSBI_SITE = $(call github,riscv,opensbi,v$(OPENSBI_VERSION))
 endif
 
 OPENSBI_LICENSE = BSD-2-Clause
+ifeq ($(BR2_TARGET_OPENSBI_LATEST_VERSION),y)
 OPENSBI_LICENSE_FILES = COPYING.BSD
+endif
 OPENSBI_INSTALL_TARGET = NO
 OPENSBI_INSTALL_STAGING = YES