imagebuilder: clean package_list

imagebuilder: clean package_list

commit 19ac879954 (imagebuilder: add package_list function) introduced
a new function 'package_list' to the imagebuilder Makefile.
Unfortunately the package list was poluted by stdout noise of the
Makefile itself as well as opkg. Redirect those outputs to stderr to
make sure that the package_list returned doesn't contain progress
info output but really only packages.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>