kdump: sh: parse elfcorehdr command line argument

A quick cut and paste from other architectures to allow SH
to parse the elfcorehdr command line argument which is required
for both is_kdump_kernel() and vmcore to function.
(the former is as yet unused on SH).

Tested compilation only

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>