reproducible: Don't look for youngest file when no source tarball

Some packages (eg. init-ifupdown) take their source files entirely from
openembedded-core, that is, they download no source tarball.
These recipes either don't use S at all (ie. it is empty at unpack time),
or they set S = WORKDIR (as in init-ifupdown).
Looking at the file timestamps in the WORKDIR causes a non-reproducible
SOURCE_DATE_EPOCH, as files taken from file:// URIs do not have
reproducible timestamps.

If S == WORKDIR, we are better to assume that there is no source tarball,
and to fall back to a fixed timestamp for the SOURCE_DATE_EPOCH.
This makes the init-ifupdown build reproducible.

(From OE-Core rev: d395bad0179037eb5d0fa4d921985c87ae13f3a4)

Signed-off-by: Douglas Royds <douglas.royds@taitradio.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

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