log: discard non standard %e format

Created by: etienne-lms

Prior this change was %e a valid format for printing error codes as strings when identified. This change removes such support and replace use of %e with use of standard format identifier and a error code to string conversion function.

With this change, any standard printf-like format comply with SCP-firmware traces implementation. fwk_err2str() helps traces to get nice string identifier for errors.

Signed-off-by: Etienne Carriere etienne.carriere@linaro.org