carl9170 firmware: fix CMake developer Warning about CMP0050

This patch fixes the numerous warnings about the unsupported use
of the SOURCE signature for the add_custom_command:

"CMake Warning (dev) at carlfw/CMakeLists.txt (add_custom_command):
  Policy CMP0050 is not set: Disallow add_custom_command SOURCE signatures.
  Run "cmake --help-policy CMP0050" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  The SOURCE signatures of add_custom_command are no longer supported.
This warning is for project developers.  Use -Wno-dev to suppress it."

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>