allow fixing timestamps when building ext4 filesystem

When building an image, make_ext4fs currently sets the timestamps in
the image to the timestamps of the source files.  Allow this time to
be overridden with a fixed value provided on the command line, to make
it easier to reproduce bit-identical images from a target_files zip.

Change-Id: I52ddab4575a334ee52404f4d5d1c61b55513c618
4 files changed