sstatesig/populate_sdk_ext: Improve unihash cache handling

sstatesig/populate_sdk_ext: Improve unihash cache handling

Copying in the bb_unihashes cache file was at best a hack and creates a number of
challenges. One is staying in sync with bitbake since it may not have saved the
most recent version of the file. A second is a determinism problem since there may
be more entries in the file than the SDK should have had access to.

To improve the situation, add code to write the data into the locked-sigs.inc file
such that even when locked-sigs aren't used, the right hash mappings are injected
by the get_cached_unihash call.

The functions in copy_buildsystem need to be updated to preserve data they're not
editting.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>