buildman: Use spaces in the board list

buildman: Use spaces in the board list

At present the board names shown with -l are separated by commas. This
makes it hard to double-click to select a particular board. Also it is not
possible to select all boards and paste them as arguments to a subsequent
buildman run, since buildman requires spaces to separate the list on the
command line, not commas.

Change the output format to use spaces instead of commas.

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