build: add noarch support

build: add noarch support

This allows to store all architecture independent packages in a single
folder instead of storing duplicates in each architecture folder.

As a result nearly 3000 less packages are build per architecture and
instead a single *noarch* job creates packages usable by all
architectures.

Built images will have an extended `distfeeds.conf` containing the feeds
separated in arch specific and *noarch*. As this will increase the time
needed for an `opkg update`, future commits could disable the option
`PER_FEED_REPO` as all default feeds are OpenWrt maintained and
build at the same time anyway.

A new *No-Arch* field in the package metadata allows the tools
`./scripts/feeds` to sort on arch dependent and independent packages via
later commits.

Signed-off-by: Paul Spooren <mail@aparcar.org>