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>