bootconfig: Mark boot_config_checksum() static

In fact, this function is only used in this file, so mark it with 'static'.

Link: http://lkml.kernel.org/r/1581852511-14163-1-git-send-email-hqjagain@gmail.com

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>