bcm63xx: image: don't add the CFE to the sercomm factory

bcm63xx: image: don't add the CFE to the sercomm factory

There is no need to include the CFE bootloader in the Sercomm factory
images.

There might be a case when this could be useful:
  - We are running the stock firmware on the first Sercomm image
  - The second partition storing the botloader was erased (unlikely)
Even in this case flashing an image without a bootlader is harmless.

Don't include the bootloader in the factory image creation and rid of the
risk of flashing factory images with an untested bootloader partition.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>