bootconfig: Update MAINTAINERS file to add tree and mailing list

Since the bootconfig related changes will be handled on linux-trace
tree, add the tree and mailing lists for EXTRA BOOT CONFIG.

Link: https://lkml.kernel.org/r/167417138436.2333752.6988808113120359923.stgit@devnote3

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