Harmonize generate_comfort_noise_enc_*fx(), configureFdCngEnc_*fx()

Bug description

The function pairs

• configureFdCngEnc_fx() <-> configureFdCngEnc_ivas_fx()
• generate_comfort_noise_enc_fx() <-> generate_comfort_noise_enc_ivas_fx()

are almost identical. They could be harmonized with little effort.