Default Menu Script

This MR adds a default menu script and adds functionality to Epicardium to load it onto the external flash if no menu.py exists yet.

Documentation/pycardium/os.rst

+.. py:module:: os
+
+``os`` - OS Functions
+=====================
+The ``os`` module allows access to a few core functionalities of Epicardium and functions found in CPythons ``os`` module.
+
+CPython-Like
+------------
+
+.. py:function:: os.unlink(path)
+
+   Unlink (remove) a file.
+
+   :param str path: The file to remove.
+
+.. py:function:: os.listdir(dir)
+
+   List contents of a directory.
+
+   :param str dir: Path to the directory to list.
+   :returns: A list of entities (files or subdirectories) in the directory
+      ``dir``.
+
+Epicardium-Specific
+-------------------
+
+.. py:function:: os.exit(ret = None)
+
+   Exit from the current app and return to the menu.
+
+
+   :param int ret: Optional return code, same semantics as Posix (``0`` means
+      success).
+   :return: This function will never return.
+
+.. py:function:: os.exec(name)
+
+   Try executing a new app, stopping the currently running one.
+
+   ``name`` is the path to either a l0dable (ending in ``.elf``) or a python
+   script (ending in ``.py``).  If the path does not lead to an executable file,
+   ``os.exec()`` will raise an exception.
+
+   :param str name: Path to new app/script/l0dable.
+   :return: This function never returns.  It can, however raise an exception.