ports: Move '.frozen' to second entry in sys.path.

In commit 86ce442607 the '.frozen' entry was
added at the start of sys.path, to allow control over when frozen modules
are searched during import, and retain existing behaviour whereby frozen
was searched before the filesystem.

But Python semantics of sys.path require sys.path[0] to be the directory of
the currently executing script, or ''.

This commit moves the '.frozen' entry to second place in sys.path, so
sys.path[0] retains its correct value (described above).

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/sys_path.py

@@ -0,0 +1,16 @@
+# test sys.path
+
+try:
+    import usys as sys
+except ImportError:
+    import sys
+
+# check that this script was executed from a file of the same name
+if "__file__" not in globals() or "sys_path.py" not in __file__:
+    print("SKIP")
+    raise SystemExit
+
+# test that sys.path[0] is the directory containing this script
+with open(sys.path[0] + "/sys_path.py") as f:
+    for _ in range(4):
+        print(f.readline())