build: use numeric-owner in ipkg-build

build: use numeric-owner in ipkg-build

To create packages the `ipkg-build` script is used which double packs
`control.tar.gz` and `data.tar.gz` to a single package. By default it's
using a verbose username instead of a numeric value for files.

Official OpenWrt images (artifacts) are created within docker containers
which do not seem to contain those verbose usernames and instead
defaults to numeric values.

This becomes a problem when rebuilding public artifacts because other
build environments may offer verbose usernames and there the created
packages is different from the official ones.

With this commit `ipkg-build` always uses numeric values for user/group
and thereby making it easier to reproduce official artifacts.

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