loader: make sure bfs debug output ends up in log

Use dprintf instead of printf so any debug output ends up in
bootloader log instead of only being displayed on-screen.
CompileDTS: use preprocessor on .dts files

Recent linux behaviour (and also copied by BSD) is to preprocess
DTS files with the C preprpocessor to enable sharing constants between
driver implementation and DTS content for more readability.
ARM: beagle: specify Beagle-XM DTS file
fdt: humble beginnings of Flattened Device Tree

This isn't really a bus_manager yet, but just minimal support so
we can get rid of hardcoded ARM SoC support from the core kernel
code.

Needs lots of work, like proper handling of #address-cells and
the like. Also, generic attribute handling, device_manager
integration, and I could go on for hours ;)
ARM: add FDT bus_manager to floppy disk image
ARM: platform: grab the FDT from the bootloader
ARM: u-boot: fixup FDT handling

If we find an FDT (either from uImage or otherwise) we make sure
we map it after mmu_init() and use kernel_args to pass it to the
kernel (so it is available at all times there).
revert accidental adding of xhci to boot floppy

I should not be doing multiple (unrelated) things at the same
time I guess ;)