userboot.so is the FreeBSD user-mode bootloader

So, this is a bit horrible but it works:

  - userboot is compiled on FreeBSD with '-target x86_64-apple-darwin14' CFLAGS
  - same for the dependencies of userboot (stand, ficl, zfs)
  - you have to use the MachO linker set header (include/xhyve/support/linker_set.h)
  - the resulting object files are linked on OS X with 'clang -dead_strip -shared -o userboot.so *.o *.So'