Dear Anil:
Good day!
Both the project management plan and the configuration management plan are parts of the project life cycle. For simpler projects, the configuration management plan might not be developed. The project manager will decide which components of the project management plan should be developed in detail, which at a high level and which should not be developed at all. However, if the configuration management plan is developed, it becomes a part of the project management plan and hence becomes a part of the project life cycle.
I am not sure where you read that the configuration management plan is not the part of the project life cycle. If you can share the reference, it would be great. However, please note that the project life cycle is a part of the overall product life cycle. Every product life cycle has a project life cycle. At a very high level, it could be understood as the project life cycle of a product develops the product design and the first product and then it ends, while the product life cycle continues till the product is produced until it is retired. The configuration management plan is used throughout the product life cycle and its use is not only limited to the project life cycle of the project.
Let me know if this resolves your confusion.
Warm regards,
Khurram