x86: Add get_ramdisk_image/size()

There are several places to find ramdisk information early for reserving
and relocating.

Use accessor functions to make code more readable and consistent.

Later will add ext_ramdisk_image/size in those functions to support
loading ramdisk above 4g.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-16-git-send-email-yinghai@kernel.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>