log: don't show function by default

The name of the function emitting a log message may be of interest for a
developer but is distracting for normal users. See the example below:

    try_load_entry() Booting: Debian

Make the default format for log messages customizable. By default show
only the message text.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

test/py/tests/test_log.py

@@ -39,6 +39,8 @@ def test_log(u_boot_console):
         Returns:
             iterator containing the lines output from the command
         """
+        output = u_boot_console.run_command('log format fm')
+        assert output == ''
         with cons.log.section('basic'):
            output = u_boot_console.run_command('log test %d' % testnum)
         split = output.replace('\r', '').splitlines()