Upgrade fdt library to the very latest

The new helpers remove a lot of boilerplace. Unfortunately some rough edges still remain (in particular issue gh#12 which renders interrupts() helper useless)

I was hoping the fdt author would address some of the issues and/or the promised fdt v2 rewrite would arrive so I could clean this up further before publishing. Alas, neither has happened and Redox feature freeze is over, so we'll have to live with those rough edges for now.