waf: use blddestdir to define task generator name

The only exception is for binaries that go in 'bin', like vehicles.
That way, partial builds follow the rule:
 - If building a vehicle or some helper that goes into 'bin', just use the
   binary name as the argument for --target. Example: `waf --target
   ArduCopter`.
 - For other binaries, the name of the directory they are placed in must be
   used. Example: `waf --target tests/test_vectors`.

Tools/ardupilotwaf/ardupilotwaf.py

@@ -89,6 +89,9 @@ def program(bld, blddestdir='bin', **kw):
 
     target = bld.bldnode.find_or_declare(blddestdir + '/' +
                                          name + '.' + bld.env.BOARD)
+    if blddestdir != 'bin':
+        name = blddestdir + '/' + name
+
     bld.program(
         target=target,
         name=name,