Modernize Makefiles.

In particular:

* Use XMOS_MAKE_PATH to pick up latest version of xcommon.
* Change target from XK-1 -> XK-1A (XK-1 target isn't included in latest
  tools release).
* Implement top level makefile by including the common Makefile.toplevel
  and change BUILD_SUBDIRS to all.
Fix used modules of app_example_uint64.
Mark module_lib_uint_64 as depending on module_lib_uint_32
Merge pull request #3 from rlsosborne/modernize_makefiles

Modernize makefiles - thanks Rich