framework: add support of log prefix when printing a trace.

Hello,

I'm using the SCP in OP-TEE and we have issues to automatically parse error logs of the SCP during runtime.

To enhance the log verbosity and be sure that it come from the SCP (and not the Linux Kernel) I added a prefix different for each log level of SCP.

I do not want to impact logs of all platform of the SCP and possibly break tests of other platform. So the log prefixes are enabled by a switch SCP_ENABLE_LOG_PREFIX (default disable). I choose to use #define to define the string of each prefix (eg : FWK_LOG_PREFIX_LOCAL) so if a platform wants to customize the prefix, it could be done in cmake platform files.

The customization of the prefix is not supported in this PR.

Thomas.