Only use front/back layer notation "F&B" for zone definitions.

This prevents false positive DRC footprint parity errors caused by commit
088e0e80a1.

pcbnew/pcb_io/kicad_sexpr/pcb_io_kicad_sexpr.cpp

@@ -1445,7 +1445,7 @@ void PCB_IO_KICAD_SEXPR::format( const FOOTPRINT* aFootprint ) const
 }
 
 
-void PCB_IO_KICAD_SEXPR::formatLayers( LSET aLayerMask, bool aEnumerateLayers ) const
+void PCB_IO_KICAD_SEXPR::formatLayers( LSET aLayerMask, bool aEnumerateLayers, bool aIsZone ) const
 {
     static const LSET cu_all( LSET::AllCuMask() );
     static const LSET fr_bk(  { B_Cu, F_Cu } );
@@ -1474,7 +1474,11 @@ void PCB_IO_KICAD_SEXPR::formatLayers( LSET aLayerMask, bool aEnumerateLayers )
         }
         else if( ( aLayerMask & cu_board_mask ) == fr_bk )
         {
-            output += ' ' + m_out->Quotew( "F&B.Cu" );
+            if( aIsZone )
+                output += ' ' + m_out->Quotew( "F&B.Cu" );
+            else
+                output += ' ' + m_out->Quotew( "*.Cu" );
+
             aLayerMask &= ~fr_bk;
         }
 
@@ -2578,7 +2582,7 @@ void PCB_IO_KICAD_SEXPR::format( const ZONE* aZone ) const
 
     // Always enumerate every layer for a zone on a copper layer
     if( layers.count() > 1 )
-        formatLayers( layers, aZone->IsOnCopperLayer() );
+        formatLayers( layers, aZone->IsOnCopperLayer(), true );
     else
         formatLayer( aZone->GetFirstLayer() );
 

pcbnew/pcb_io/kicad_sexpr/pcb_io_kicad_sexpr.h

@@ -477,7 +477,7 @@ private:
 
     void formatLayer( PCB_LAYER_ID aLayer, bool aIsKnockout = false ) const;
 
-    void formatLayers( LSET aLayerMask, bool aEnumerateLayers ) const;
+    void formatLayers( LSET aLayerMask, bool aEnumerateLayers, bool aIsZone = false ) const;
 
     friend class FP_CACHE;