Change from blackbox to whitebox annotation.

Yosys's treatment of blackbox attributes's now includes stripping all contents
within the blackbox.  The whitebox attribute is what should be used to
explicitly define contents of a blackbox.

This PR also updates Yosys (as of 4 days ago at b6345b1).

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>