These two variables were missing in the definition of
DEFAULT_DEVICE_VARS which caused them to contain wrong values, messing
up the resulting JSON files.
This patch adds the two variables IMAGE_PREFIX and DEVICE_PACKAGES to
DEFAULT_DEVICE_VARS.
Suggested-by: Baptiste Jonglez <git@bitsofnetworks.org>
Signed-off-by: Paul Spooren <mail@aparcar.org>