There are two components in Isar who uses isar-apt:
 - buildchroot
 - images

At the moment the only buildchroot has dapendency from isar-apt, this
patch adds it also for images.

Signed-off-by: Alexander Smirnov <asmirnov@ilbers.de>
Reported-by: Henning Schild <henning.schild@siemens.com>