Fix FIT image detection for larger image files. (MR: !75)

Use manual magic comparison for FIT images (device tree blob) to work
around a bug in libmagic that results in device tree blob files not
correctly being identified when the structure section is larger than 1MB.

libmagic bug report: https://bugs.astron.com/view.php?id=232



Signed-off-by: Chris Lamb <lamby@debian.org>