configs: sandbox64: Increase console record size to 0x6000

Align the sandbox64 defconfig with sandbox defconfig. Increase the
console record size. This fixes ut_bootstd_bootflow_cmd_scan_e .

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