Set time and umask on multi-dtb fit images to ensure reproducibile builds.

Set time and umask on multi-dtb fit images to ensure reproducibile builds.

When building compressed (lzop, gzip) multi-dtb fit images, the
compression tool may embed the time or umask in the image.

Work around this by manually setting the time of the source file using
SOURCE_DATE_EPOCH and a hard-coded 0600 umask.

With gzip, this could be accomplished by using -n/--no-name, but lzop
has no current workaround:

  https://bugs.debian.org/896520

Signed-off-by: Vagrant Cascadian <vagrant@debian.org>