build - Support concurrent SUBDIR traversal, fix make depend bug

build - Support concurrent SUBDIR traversal, fix make depend bug

* Support concurrent SUBDIR traversals during a make -j N.

  Create individual targets for each target/directory combination so Make
  will run them in parallel when possible.

* SUBDIR_ORDERED, if it exists, indicates the subset of SUBDIR which must
  be ordered.  Any directories not mentioned can run concurrently.

* If SUBDIR_ORDERED does not exist at all then all directories default
  to being strongly ordered and will NOT run concurrently.

* Try to make sure that all include files and generated source files
  are generated before running the depend core.