ramips: fix build error on Airlink AR670W

The 'KERNEL' is not referenced by other objects, so double '$$' will
cause shell unable to parse the variable 'BLOCKSIZE':
  dd ... bs=$(BLOCKSIZE) conv=sync
  bash: line 1: BLOCKSIZE: command not found

Fixes: 09a0efbe83(ramips: set default BLOCKSIZE to 64k for nor flash devices)
Signed-off-by: Shiji Yang <[email protected]>

target/linux/ramips/image/rt288x.mk

@@ -15,7 +15,7 @@ define Device/airlink101_ar670w
   DEVICE_VENDOR := Airlink
   DEVICE_MODEL := AR670W
   IMAGE_SIZE := 3840k
-  KERNEL := $(KERNEL_DTB) | pad-to $$$$(BLOCKSIZE)
+  KERNEL := $(KERNEL_DTB) | pad-to $$(BLOCKSIZE)
   IMAGES += factory.bin
   IMAGE/factory.bin := $$(sysupgrade_bin) | check-size | \
 	wrg-header wrgn16a_airlink_ar670w