ath79: mark netgear variables as device specific

The variables are used in image build recipes and need to be marked as
per devices vars to be stored individual per image define. Otherwise
the last defined variable will be used for all boards.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ath79/image/common-netgear.mk

@@ -1,3 +1,5 @@
+DEVICE_VARS += NETGEAR_KERNEL_MAGIC NETGEAR_BOARD_ID NETGEAR_HW_ID
+
 define Build/netgear-squashfs
 	rm -rf $@.fs $@.squashfs
 	mkdir -p $@.fs/image