Our initramfs images are supposed to be small and don't have a
/etc/opkg folder so attempting to put feed config in there will
fail. Reset the FEED_URIS that come from our local.conf.
Splitt of the actual rescue root filesystem to
image-rauc-rescue-initramfs and build image-rauc-ubi using
kernel+initramfs (like normal rauc slots).
Signed-off-by: Jan Luebbe <jluebbe@debian.org>