s390: add support for --reuse-cmdline

--reuse-cmdline reads the command line of the currently
running kernel from /proc/cmdline and uses that for the
kernel that should be kexec'd.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Simon Horman <horms@verge.net.au>