base-files: add ARCH_PACKAGES to openwrt_release and os-release

Knowing the package architecture at runtime can be useful, e.g. to
configure opkg repository URLs. The value of ARCH_PACKAGES ("%A" in
VERSION_SED) as added to openwrt_release (as DISTRIB_ARCH) and os-release
(as LEDE_ARCH).

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>