brcm63xx: fix image generation with offsets/blocksizes != 64K

Defer the CFE_EXTRAs evaluation to allow overriden BLOCK_SIZE and
IMAGE_OFFSET take effect, and replace the unused IMAGE_SIZE with
IMAGE_OFFSET in DEVICE_VARS.

This fixes image generation for targets using different offsets or
blocksizes.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

target/linux/brcm63xx/image/bcm63xx.mk

@@ -28,9 +28,9 @@ define Device/bcm63xx
   FLASH_MB := 4
   CFE_BOARD_ID :=
   CFE_CHIP_ID :=
-  CFE_EXTRAS := --block-size $$(BLOCK_SIZE) --image-offset $$(if $$(IMAGE_OFFSET),$$(IMAGE_OFFSET),$$(BLOCK_SIZE))
+  CFE_EXTRAS = --block-size $$(BLOCK_SIZE) --image-offset $$(if $$(IMAGE_OFFSET),$$(IMAGE_OFFSET),$$(BLOCK_SIZE))
 endef
-DEVICE_VARS += BLOCK_SIZE FLASH_MB IMAGE_SIZE
+DEVICE_VARS += BLOCK_SIZE FLASH_MB IMAGE_OFFSET
 DEVICE_VARS += CFE_BOARD_ID CFE_CHIP_ID CFE_EXTRAS
 
 define Device/bcm63xx_netgear