Split the string importer from the rest of the mpy parsing

Split the string importer from the rest of the mpy parsing
and importing stuff to avoid some confusion.

sim/pyconfig/SConscript:
    Split the string importer from the rest of the importer code.
    The importer.py code can be embedded like m5config.py
sim/pyconfig/m5config.py:
    import what we need from importer

--HG--
extra : convert_revision : 9d57f43381b55e717b5b10adfb8f0a522280ac57