TLS 1.3: Improve the key generation functions.

[yuhaoth]

Suggested enhancement

• [x] Change early_secrets to local variable
• [ ] Re-consider about tls13_hs_secrets
• [x] Change functions to static. There are many functions not used by other module, we should change them to local.

And so on.

Justification

See https://github.com/Mbed-TLS/mbedtls/pull/6538#discussion_r1030216900

The key generation of TLS 1.3 is changed for time being. Some generation functions is not used outside. And some variables can be local variable. That improvement is target to reduce the code size and memory space, improve readability for current code.

[yanrayw]

Create a checklist to match issues with PRs.

• [x] Change early_secrets to local variable #6773
• [x] Change functions to static. There are many functions not used by other module, we should change them to local. #6791
• [ ] Re-consider about tls13_hs_secrets

[yanrayw]

This issue is resolved by small PRs. #6773 is still in review. The PR to handle Re-consider about tls13_hs_secrets is still not uploaded.