config: ExtraDebs: replace with DebianExtraPackages

config: ExtraDebs: replace with DebianExtraPackages

`ExtraDebs' is a silly name.  Also the semantics are rather
inflexible; we might want to install specific packages rather than
the contents of a whole directory.

And, document it.

This variable has only just been introduced, so hopefully it is OK to
replace and rename it now without causing too much disruption.

(osstest's own production runs each use their own copy of the config,
so they will be fine.  It's just downstreams, or users whose config is
set up to use one not in their own tree, who will be affected.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
------------------------------

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>