wic: make ext2/3/4 images reproducible

Ext2/3/4 FS contains not only mtime, but also ctime, atime and crtime.
Currently, all the files are being added into the rootfs image using
mkfs -d functionality which affects all the timestamps excluding mtime.
This patch ensures these timestamps inside the FS image equal to
the SOURCE_DATE_EPOCH if it is set.

Signed-off-by: Sergei Zhmylev <s.zhmylev@yadro.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>