bootconfig: Use parse_args() to find bootconfig and '--'

bootconfig: Use parse_args() to find bootconfig and '--'

The current implementation does a naive search of "bootconfig" on the kernel
command line. But this could find "bootconfig" that is part of another
option in quotes (although highly unlikely). But it also needs to find '--'
on the kernel command line to know if it should append a '--' or not when a
bootconfig in the initrd file has an "init" section. The check uses the
naive strstr() to find to see if it exists. But this can return a false
positive if it exists in an option and then the "init" section in the initrd
will not be appended properly.

Using parse_args() to find both of these will solve both of these problems.

Link: https://lore.kernel.org/r/202002070954.C18E7F58B@keescook
Fixes: 7495e0926fdf3 ("bootconfig: Only load bootconfig if "bootconfig" is on the kernel cmdline")
Fixes: 1319916209ce8 ("bootconfig: init: Allow admin to use bootconfig for init command line")
Reported-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>