lantiq: fix setting SOC to DEFAULT_SOC

This adds the missing assignment of DEFAULT_SOC to the SOC variable
by default.

Fixes: 09ee51c614 ("lantiq: define SOC only once for uniform targets")

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/lantiq/image/Makefile

@@ -63,6 +63,7 @@ define Device/Default
   KERNEL_INITRAMFS := kernel-bin | append-dtb | lzma | uImage lzma
   FILESYSTEMS := squashfs
   IMAGE_SIZE :=
+  SOC := $(DEFAULT_SOC)
   DEVICE_DTS = $$(SOC)_$(1)
   SUPPORTED_DEVICES := $(subst _,$(comma),$(1))
   IMAGES := sysupgrade.bin