x86: x86-common.h: Add generic FS commands

This patch adds the generic FS commands (ls, load) to all x86 boards.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Miao Yan <yanmiaobest@gmail.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Bin Meng <bmeng.cn@gmail.com>