linux: fix double LINUX_APPEND_DTB

Introduced by 5b33e0b6a (linux: handle new dtb location since 3.8-rc1 for
appended dtbs).

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

linux/linux.mk

@@ -202,7 +202,6 @@ endif
 endif
 
 ifeq ($(BR2_LINUX_KERNEL_APPENDED_UIMAGE),y)
-define LINUX_APPEND_DTB
 define LINUX_APPEND_DTB
 	# dtbs moved from arch/$ARCH/boot to arch/$ARCH/boot/dts since 3.8-rc1
 	cat $(wildcard $(addprefix $(KERNEL_ARCH_PATH)/boot/,\