bcm47xx: Fix Netgear WNR3500L v2 image format

CFE on these devices expects to find the kernel compressed with LZMA but
with no dictionnary and no loader, adjust the image generation
accordingly.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

target/linux/bcm47xx/image/mips74k.mk

@@ -460,6 +460,8 @@ define Device/netgear_wnr3500l-v2
   $(Device/netgear)
   NETGEAR_BOARD_ID := U12H172T00_NETGEAR
   NETGEAR_REGION := 1
+  KERNEL_NAME = vmlinux-nodictionary.lzma
+  IMAGE/chk := append-rootfs | trx-without-loader | netgear-chk
 endef
 TARGET_DEVICES += netgear_wnr3500l-v2