Support tracing in config.mk when enabled

Use -finstrument-functions when tracing is enabled (make FTRACE=1).
Tracing is not currently supported by SPL even if sufficient memory is
available.

When tracing is enabled, we #define FTRACE. This can be used by
board config files to conditionally enable the tracing options.

Signed-off-by: Simon Glass <sjg@chromium.org>