x86 boot: document for 32 bit boot protocol

This patch documents the 32-bit boot protocol of x86. It has been used
by Kexec and LinuxBIOS. This patch is based on the proposal of Peter
Anvin.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>