Moves most of the python code used for dealing with the source date
epoch to library code.
(From OE-Core rev: a7ede90955bc0c8bec1cbb3cab498ef2583b2f4e)
Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>