[PATCH] kdump: x86_64: add elfcorehdr command line option

- elfcorehdr= specifies the location of elf core header stored by the
  crashed kernel.  This command line option will be passed by the kexec-tools
  to capture kernel.

Changes in this version :

- Added more comments in kernel-parameters.txt and in code.

Signed-off-by: Murali M Chakravarthy <muralim@in.ibm.com>
Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
Cc: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>