x86: theadorable-x86: Add header file for common defines and env

This patch adds the common header include file theadorable-x86-common.h
for the theadorable-x86 targets to define all common options and the
default environment.

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