update: consolidated flat device tree code

Changes:
- converted some ints to "unsigned", since gcc was generating warnings
and negative lengths don't make sense in the flat data structure.
- added ft_set_prop(), which doesn't yet resize properties but will in
the future.
- added an explicit copyright statement.

Still haven't looked to merge Matt's changes (because I'm busy, the
current code works for me, and they haven't been accepted in u-boot
yet).

Current users: Xen
Future users: Mark's Linux bootwrapper code
Potential users: kexec, u-boot

-- 
Hollis Blanchard
IBM Linux Technology Center