Disable AVR32 U-Boot until working

target/device/Atmel/u-boot/Config.in

@@ -1,6 +1,6 @@
 config BR2_TARGET_UBOOT
 	bool "Das U-Boot Boot Monitor"
-	depends on BR2_TARGET_AT91 || BR2_TARGET_AVR32
+	depends on BR2_TARGET_AT91
 	depends on !BR2_TARGET_AT91SAM9260PF
 	default n
 	help