fdt_file overload

We're setting fdt_file in conf/boards as well as in root.sh's setup_kernel(). We also mangle the devkit's fdt_file in setup_uboot(). None of this is needed for the devkit so we should clean it up to make future expansion possible (e.g. adding the phone) without getting headaches ;)

@angus.ainslie could you have a look since? We should like separate flash-kernel and non-flash kernel based boards (putting the later on to the side and making these the exception instead of the rule)

Corrections: we're only setting fdt_source in root.sh but we should still unmangel build_uboot() and build_kernel() to only do the necessary steps for the devkit.