kexec-tools: s390: Reset kernel command line on syscall fallback

The command line is duplicated on s390 if kexec_file_load(2) is not
implemented. That's because the corresponding variable is not reset
to an empty string before re-parsing the kexec command line.

Fixes: 9cf72127 ("Reset getopt before falling back to legacy syscall")
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Signed-off-by: Simon Horman <horms@verge.net.au>