With the new `SOURCE` argument of `get_source_date_epoch` it is possible
to set package timestamps based on actual package changes rather thane
$TOPDIR changes.
This commit adds a new variable PKG_SOURCE_DATE_EPOCH which is used by
the `ipkg` build script. As a fallback the existing SOURCE_DATE_EPOCH is
used or as last resort the current time.
The redundant checks for `.git/` and `.svn/` are removed.
Signed-off-by: Paul Spooren <mail@aparcar.org>
