tools, fit_info: increase buffer for command name

currently the buffer for command name is 50 bytes only. If using
fit_info with long absolute paths, this is not enough, so raise
it to 256 (as it is in fit_check_sign)

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>