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

Support for compressed fit-dtb.blob was added in:

  commit 95f4bbd5 ("lib: fdt: Allow LZO and GZIP DT compression in
  U-Boot")

When building compressed (lzop, gzip) fit-dtb.blob 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

This is essentially the same fix applied to multi-dtb fit SPL images in:

  commit 8664ab7d

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

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