build directory can now be renamed

The autogenerated header files have been moved about, and an extra
include dir has been added, which means you can give a custom
BUILD=newbuilddir option to make, and everything "just works"

Also tidied up the way the different Makefiles build their include-
directory flags

py/qstr.h

@@ -8,7 +8,6 @@ enum {
     MP_QSTR_NULL = 0, // indicates invalid/no qstr
     MP_QSTR_ = 1, // the empty qstr
 #define Q(id, str) MP_QSTR_##id,
-// TODO having 'build/py.' here is a bit of a hack, should take config variable from Makefile
 #include "build/py/qstrdefs.generated.h"
 #undef Q
     MP_QSTR_number_of,