cmake: Finer granularity in configuring doc generation

BUILD_DOCUMENATION is now a top-level switch to turn on/off doc generation.

When BUILD_DOCUMENTATION=ON, individual components may be turned ON/OFF.
These default to the state of ${BUILD_DOCUMENTATION}.
    - BUILD_LIBBLADERF_DOCUMENTATION      (libbladeRF Doxygen docs)
    - BUILD_BLADERF_CLI_DOCUMENTATION    (bladeRF-cli man page)
    - BUILD_BLADERF_FLASH_DOCUMENTATION  (bladeRF-flash man page)