Misc updates to ethosu_log

Replace empty log macros with calls to a new LOG_COMMON_NOP macro, with
the intention of avoiding unused variable/parameter warnings where the
variables/parameters are currently only used for logging. The if check
will normally be optimized away.

Change formatting of error messages, prepend the file name and line
instead of appending it, to allow for newlines in error messages.

Add a check for __FILE_NAME__ macro and use that instead of strrchr
function, when available.

Change-Id: I88a868c04b9db112fcec2d6209f9943c85925be7
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>