tools: config-create: rename parse_cmdline to parse_kernel_cmdline

`parse_cmdline` is quite missleading in the context of a script with
command-line parameters. Thus better name it for what it does, parse the
kernel command line.

Signed-off-by: Benjamin Block <bebl@mageta.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>