gen-lockedsig-cache: copy correct native sstate into ext SDK
When constructing the sstate-cache directory for the extensible SDK,
we were copying in any matching native sstate packages, and as the
signature doesn't actually change when the distro changes (since
NATIVELSBSTRING is just a path separator for the artifacts and is not
part of the signature) we ended up copying duplicated packages when the
distro changed e.g. upon host distro upgrade. Only search in the
NATIVELSBSTRING-named subdirectory for native packages and the issue
goes away.
Fixes [YOCTO #8885].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
