py: Rework QSTR extraction to work in simple and obvious way.

When there're C files to be (re)compiled, they're all passed first to
preprocessor. QSTR references are extracted from preprocessed output and
split per original C file. Then all available qstr files (including those
generated previously) are catenated together. Only if the resulting content
has changed, the output file is written (causing almost global rebuild
to pick up potentially renumbered qstr's). Otherwise, it's not updated
to not cause spurious rebuilds. Related make rules are split to minimize
amount of commands executed in the interim case (when some C files were
updated, but no qstrs were changed).

py/qstr.h

@@ -37,9 +37,11 @@
 
 // first entry in enum will be MP_QSTR_NULL=0, which indicates invalid/no qstr
 enum {
+#ifndef __QSTR_EXTRACT
 #define QDEF(id, str) id,
 #include "genhdr/qstrdefs.generated.h"
 #undef QDEF
+#endif
     MP_QSTRnumber_of, // no underscore so it can't clash with any of the above
 };