setup_board: seed make.conf.user with tips

It's easy to do the wrong thing in this file.  Seed it with some useful
comments to try and head that off.

BUG=None
TEST=`rm /etc/make.conf.user && ./setup_board --board=lumpy` setup a comment filled file
TEST=`echo '#' > /etc/make.conf.user && ./setup_board --board=lumpy` left existing file alone

Change-Id: I23405818f03972d5bb930f4c4b80912c2c67b3de
Reviewed-on: https://gerrit.chromium.org/gerrit/44022
Reviewed-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed