Simplify and correct variable assignments

- Remove asgnMaybeAppend, it was only used to indicate
  that the asignment needs a setDefault. But really, all
  types of variable assignments need setDefault if they're
  self-referential.
- Remove local_append/local_set_default because there was
  no implementation for them written in product_config.rbc
  anyways.
- Correct productConfigVariable.emitDefined using the global
  variables instead of the product config variables.
- Emit setDefaults for all types of assignments if they're
  self referential.

Bug: 222737841
Test: go test
Change-Id: I06a0f90f16d5900049d473281e6d5ef5e93e67da

mk2rbc/mk2rbc.go

@@ -50,15 +50,12 @@ const (
 	soongNsPrefix = "SOONG_CONFIG_"
 
 	// And here are the functions and variables:
-	cfnGetCfg          = baseName + ".cfg"
-	cfnMain            = baseName + ".product_configuration"
-	cfnBoardMain       = baseName + ".board_configuration"
-	cfnPrintVars       = baseName + ".printvars"
-	cfnWarning         = baseName + ".warning"
-	cfnLocalAppend     = baseName + ".local_append"
-	cfnLocalSetDefault = baseName + ".local_set_default"
-	cfnInherit         = baseName + ".inherit"
-	cfnSetListDefault  = baseName + ".setdefault"
+	cfnGetCfg         = baseName + ".cfg"
+	cfnMain           = baseName + ".product_configuration"
+	cfnBoardMain      = baseName + ".board_configuration"
+	cfnPrintVars      = baseName + ".printvars"
+	cfnInherit        = baseName + ".inherit"
+	cfnSetListDefault = baseName + ".setdefault"
 )
 
 const (
@@ -571,11 +568,7 @@ func (ctx *parseContext) handleAssignment(a *mkparser.Assignment) []starlarkNode
 	case "=", ":=":
 		asgn.flavor = asgnSet
 	case "+=":
-		if asgn.previous == nil && !asgn.lhs.isPreset() {
-			asgn.flavor = asgnMaybeAppend
-		} else {
-			asgn.flavor = asgnAppend
-		}
+		asgn.flavor = asgnAppend
 	case "?=":
 		asgn.flavor = asgnMaybeSet
 	default: