log: Allow padding of the function name

At present when function names are logged, the output is a little hard to
read since every function is a different length. Add a way to pad the
names so that the log messages line up vertically. This doesn't work if
the function name is very long, but it makes a big difference in most
cases.

Use 20 characters as a default since this covers the vast majority of
functions.

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