imagebuilder: fix DEFAULT_PACKAGES handling

imagebuilder: fix DEFAULT_PACKAGES handling

DEFAULT_PACKAGES handling was moved in commit 40be892a020e
("imagebuilder: move handling of DEFAULT_PACKAGES into shareable place")
to `include/default-packages.mk`, but they weren't removed from
ImageBuilder's Makefile, so lets remove it now.

Once removed, I've noticed, that it stopped working as there is
target.mk included later in that file, overriding the DEFAULT_PACKAGES
again, so moved it after this target.mk include.

Fixes: 40be892a020e ("imagebuilder: move handling of DEFAULT_PACKAGES into shareable place")
Signed-off-by: Petr Štetiar <ynezz@true.cz>