tools/bootconfig: Add list option

Add list option (-l) to show the bootconfig in the list style.
This is same output of /proc/bootconfig. So users can check
how their bootconfig will be shown in procfs. This will help
them to write a user-space script to parse the /proc/bootconfig.

Link: https://lkml.kernel.org/r/159704849087.175360.8761890802048625207.stgit@devnote2

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