local.conf is the first thing anyone new to the project sees. Over time it
has built up a ton cruft and isn't even accurate in places.

This patch:

* Moves things to local.conf.sample.extended if a new user is unlikely
  to need to immediately care about the options
* Reorders the file to be more intuitive to a new user
* Moves certain default values to default-distrovars.inc in cases where
  most users wouldn't want to change the value
* Adds large blocks of text to explain what an option does. There have
  been too many cases of a user not realising what some of these
  settings do and how they can use them to their advantage (like DL_DIR
  for example).

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>