[FEATURE] Use safer string manipulation functions in system/settings

[jeanthom]

Is your feature request related to a problem? Please describe.

The system/settings module/library uses unsafe C string manipulation functions (eg. strcmp() or strlen()) on potentially user-accessible data, creating a security hazard.

Describe the solution you'd like

The maximum key/value lengths are known at compile time (Kconfig parameters), meaning we could use safer strnlen() and strncmp() functions.

Describe alternatives you've considered

No response

Verification

• [x] I have verified before submitting the report.