build: suppress error output in feeds.mk

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>

include/feeds.mk

@@ -9,7 +9,7 @@
 -include $(TMP_DIR)/.packageauxvars
 
 FEEDS_INSTALLED:=$(notdir $(wildcard $(TOPDIR)/package/feeds/*))
-FEEDS_AVAILABLE:=$(sort $(FEEDS_INSTALLED) $(shell $(SCRIPT_DIR)/feeds list -n))
+FEEDS_AVAILABLE:=$(sort $(FEEDS_INSTALLED) $(shell $(SCRIPT_DIR)/feeds list -n 2>/dev/null))
 
 PACKAGE_SUBDIRS=$(PACKAGE_DIR)
 ifneq ($(CONFIG_PER_FEED_REPO),)