x86: adjust ramdisk load address

By default, ramdisk load address is defined to 02000000 in env string.
When loading bzImage to 01000000 (default address), there's a chance that
the ramdisk header would be overwritten by the kernel. Thus increase the
gap and make ramdisk load at 04000000 by default and also this patch
introduces a new configuration item CONFIG_RAMDISK_ADDR for this purpose

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>