command: Move command completion temporary buffer to stack

The command completion temporary buffer seems to be only
used by the argv tokenizer, move it to stack. This saves
2 kiB from the binary size (depends on configuration) per:
$ aarch64-linux-gnu-readelf -s u-boot | sort -n -k 3

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>