tools/bootconfig: Make all functions static

Make all functions static except for main(). This is just a cleanup.

Link: https://lkml.kernel.org/r/159704850135.175360.12465608936326167517.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>