py/py.mk: Split list of uPy sources into core and extmod files.

If a port only needs the core files then it can now use the $(PY_CORE_O)
variable instead of $(PY_O).  $(PY_EXTMOD_O) contains the list of extmod
files (including some files from lib/). $(PY_O) retains its original
definition as the list of all object file (including those for frozen code)
and is a convenience variable for ports that want everything.