distro-feed-configs: Make list of archs configurable

The list of architectures for which to emit feed configurations is now taken
from the DISTRO_FEED_ARCHS variable. The default value for this variable is the
list of architectures which was previously used. Thus in the default case
nothing is changed, but DISTRO_FEED_ARCHS can now be overridden in local.conf.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>