Define Soong phony rules in Make

To support dist-for-goals in Soong, we need to define all phony rules
in Make so that dist-for-goals can insert additional dependencies on
them.  Collect all the phony rules in phonySingleton and write them
out as Make rules when Soong is embedded in Make, or as blueprint.Phony
rules when Soong is run standalone.

Test: m checkbuild
Change-Id: I68201eff30744b0f487fc4f11f033767b53a627d
diff --git a/android/writedocs.go b/android/writedocs.go
index 7262ad8..9e43e80 100644
--- a/android/writedocs.go
+++ b/android/writedocs.go
@@ -69,9 +69,5 @@
 	})
 
 	// Add a phony target for building the documentation
-	ctx.Build(pctx, BuildParams{
-		Rule:   blueprint.Phony,
-		Output: PathForPhony(ctx, "soong_docs"),
-		Input:  docsFile,
-	})
+	ctx.Phony("soong_docs", docsFile)
 }