Allow unit-cover target for make to filter by package

Previous to this commit, running `make unit-cover pkg=xx`
would ignore the selected package and run unit tests and
coverage for all packages.
After this commit, the package selected with pkg= is the
only one that is tested and coverage output generated for.
If no pkg is selected, the default is as before, all pkgs.

Makefile

@@ -126,7 +126,8 @@ unit: btcd
 
 unit-cover: $(GOACC_BIN)
 	@$(call print, "Running unit coverage tests.")
-	$(GOACC_BIN) $$(go list ./... | grep -v lnrpc) -- -test.tags="$(DEV_TAGS) $(LOG_TAGS)"
+	$(GOACC_BIN) $(COVER_PKG) -- -test.tags="$(DEV_TAGS) $(LOG_TAGS)"
+
 
 unit-race:
 	@$(call print, "Running unit race tests.")