Legacy dts location support

This patch modifies `freebsd_install_fdt()` such that it autodetects whether fdt files are in machine-specific subdirs, as they are in 11-series trees, or they are all in the fdt base dir, as they are in 10-series trees.  This allows crochet to be used with both 10- and 11-series FreeBSD source trees.

As `freebsd_install_fdt()` now autodetects the fdt location, the machine-specific prefixes have been removed from .dts filenames in all calls to that routine.

This is the patch that I referred Glen to on 7 March.