x86: Add ramboot and nfsboot commands in x86-common.h

It is very common in the debug stage to test U-Boot loading a linux
kernel. The commands to boot linux kernel with ramdisk and nfs as the
root are common to all x86 targets, so it makes sense to add them as
the U-Boot default environment in x86-common.h.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chroimum.org>