Source /etc/default/timestamp if present. This allows the rootfs to be
read-only / store the file in a persistant location if the whole rootfs
is updated.
(From OE-Core rev: b4de37b7176630d9585b12c50d537cd561c7aeb9)
Signed-off-by: Jeroen Hofstee <jhofstee@victronenergy.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>