Fix AVR32 ext toolchain

target/device/Config.in.toolchain

@@ -1,7 +1,7 @@
 if BR2_TOOLCHAIN_EXTERNAL_SOURCE
 choice
 	prompt "Source location:"
-	default BR2_TOOLCHAIN_ATMEL_AVR32 if BR2_avr32
+	default BR2_TOOLCHAIN_ATMEL_AVR32_4_2_1 if BR2_avr32
 	help
 	  Select whether to use the toolchain built by the buildroot
 	  system or an external pre-built toolchain.