local.conf.sample: comment out the root-with-empty-password setting

local.conf.sample: comment out the root-with-empty-password setting

This was not a good default; the autobuilder relied on it, but this
has been fixed, there's now a separate fragment as well, and testimage
has a check for the needed image features. Let's take this out and
be more secure by default everywhere.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>