reproducible_builds: SOURCE_DATE_EPOCH should not be 0

A SOURCE_DATE_EPOCH of 0 might be misinterpreted by namely rpm as no SDE.

 e.g.:
 char *srcdate = getenv(SOURCE_DATE_EPOCH);
 if (srcdate && rpmExpandNumeric(%{?clamp_mtime_to_source_date_epoch}))

Solve this by:
- providing a SOURCE_DATE_EPOCH_FALLBACK variable != 0
- changing defaults to use it
- using SOURCE_DATE_EPOCH_FALLBACK also when reading 0 out of sstate-cache

Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>