busybox: show reproducible timestamp

busybox: show reproducible timestamp

On login busybox shows a timestamp per default contianing the build
date. Since the build date isn't reproducible per default this behaviour
was disabled by default via 34df4d40 "busybox: disable timestamp in
version".

This commit modifies busybox so that the printed timestamp reproducible
using SOURCE_DATE_EPOCH and therefore shouldn't be disabled anymore.

Before:

    BusyBox v1.33.1 () built-in shell (ash)

After:

    BusyBox v1.33.1 (2021-05-13 09:34:34 UTC) built-in shell (ash)

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