Add pkg-map element.

Adds a new pkg-map element which can be used to install
packages based on an in element 'pkg-map' JSON config
file format.

As part of this change we expose DIB's IMAGE_ELEMENT variable
so that we can have access to the element names which are being
installed in a clean manner.

This change is intended to decouple elements from DIB
and allow new elements to support multiple distributions
(with various package naming schemes) without having to
constantly maintain DIB's various bin/map-packages files.

Change-Id: Ic0a061995e2ae708c95a535cba6237bff58f7d93