at91sam9260dfc_defconfig: fix linux defconfig name

The 9260 defconfig got renamed to at91sam9260_9g20_defconfig in kernel
3.11.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

configs/at91sam9260dfc_defconfig

@@ -16,4 +16,4 @@ BR2_TARGET_UBOOT_BOARDNAME="at91sam9260ek_dataflash_cs0"
 
 # Kernel
 BR2_LINUX_KERNEL=y
-BR2_LINUX_KERNEL_DEFCONFIG="at91sam9260"
+BR2_LINUX_KERNEL_DEFCONFIG="at91sam9260_9g20"