x86, doc: Add a formal bootloader ID for kexec-tools

Add bootloader id D for kexec-tools (it is kexec-tools, not the kexec
system call, that builds the data structures, therefore it is
kexec-tools which is the "bootloader" for this purpose.)
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/50C95832.5030306@zytor.com

Documentation/x86/boot.txt

@@ -381,6 +381,7 @@ Protocol:	2.00+
 	A  Gujin
 	B  Qemu
 	C  Arcturus Networks uCbootloader
+	D  kexec-tools
 	E  Extended		(see ext_loader_type)
 	F  Special		(0xFF = undefined)
        10  Reserved