opkg: Extend 'opkg list' command to optionally display package size

'opkg list' command only displays the available packages' name, version and
description. It would be useful to also see the approximate size of the
available package.

This patch extends "opkg list" command with "--size" to optionally show also
the *.ipk size.
* Default behaviour is to list the available packages as earlier:
  "name - version - description"
* with "--size" the output of is "name - version - size - description".

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

SVN-Revision: 46980