build: add buildinfo files for reproducibility

build: add buildinfo files for reproducibility

generate feeds.buildinfo and version.buildinfo in build dir after
containing the feed revisions (via ./scripts/feeds list -sf) as well as
the current revision of buildroot (via ./scripts/getver.sh).

With this information it should be possible to reproduce any build,
especially the release builds.

Usage would be to move feeds.buildinfo to feeds.conf and git checkout the
revision hash of version.buildinfo.

Content of feeds.buildinfo would look similar to this:

    src-git routing https://git.openwrt.org/feed/routing.git^bf475d6
    src-git telephony https://git.openwrt.org/feed/telephony.git^470eb8e
    ...

Content of version.buildinfo would look similar to this:

    r10203+1-c12bd3a21b

Without the exact feed revision it is not possible to determine
installed package versions.

Also rename config.seed to config.buildinfo to follow the recommended
style of https://reproducible-builds.org/docs/recording/

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