Seriallog set to `SERIAL_LOG_LEVEL` at boot

[s-hadinger]

Description:

Seriallog level is set to SERIAL_LOG_LEVEL at boot instead of hard-coded LOG_LEVEL_INFO, which allows to set a different level for debug purposes.

There is no change for default binaries.

Checklist:

• [x] The pull request is done against the latest development branch
• [x] Only relevant files were touched
• [x] Only one feature/fix was added per PR and the code change compiles without warnings
• [x] The code change is tested and works with Tasmota core ESP8266 V.2.7.6
• [x] The code change is tested and works with Tasmota core ESP32 V.3.0.0
• [x] I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass