IB: use online repositories

Change the IB packaging to only embed libc, kernel and kmod packages by default
and generate repositories.conf to refer to the remote package repositories.

Introduce a new config option CONFIG_IB_STANDALONE which restores the old
behaviour of building self contained IB archives.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45772 3c298f89-4303-0410-b956-a3cf2f4a3e73