Add .blackbox support to blif front-end

Signed-off-by: Clifford Wolf <clifford@clifford.at>

frontends/blif/blifparse.cc

diff --git a/frontends/blif/blifparse.cc b/frontends/blif/blifparse.cc
index d17cacf..bfcfad7 100644
--- a/frontends/blif/blifparse.cc
+++ b/frontends/blif/blifparse.cc
@@ -174,6 +174,12 @@
 			if (module == nullptr)
 				goto error;
 
+			if (!strcmp(cmd, ".blackbox"))
+			{
+				module->attributes["\\blackbox"] = RTLIL::Const(1);
+				continue;
+			}
+
 			if (!strcmp(cmd, ".end"))
 			{
 				for (auto &wp : wideports_cache)