AxROM - support junko that has 16k prg by mirroring the block into both the 8x0000 and 0xC0000 regions. I don't know if this is correct but it matches both fceux and nestopia behavior

BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/AxROM.cs

@@ -65,6 +65,11 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 
 		public override byte ReadPRG(int addr)
 		{
+			if (Cart.prg_size ==  16)
+			{
+				return ROM[(addr & 0x3FFF) | prg << 15];
+			}
+
 			return ROM[addr | prg << 15];
 		}