If no feed.conf or feeds.conf.default is found on image generation with
the imagebuilder we always get the following message "Unable to open
feeds configuration at <dir>/scripts/feeds line 48." on std error.
To get rid off this needless warning on image generation with the
imagebuilder supress the output in feeds.mk.
Signed-off-by: Florian Eckert <fe@dev.tdt.de>