console: shell: Reduce stack usage of tab completion

Print commands names when matched instead of first creating
completion string in buffer. Common part of commands names
can only be reduced when matching further commands so only
first match needs to be kept for altering command line.

Change-Id: I5a9d2eb171ef4d31da9cbe6e93b577457b13c5a2
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>