esp8266: Change to use new generic VFS sub-system.

The VFS sub-system supports mounting of an arbitrary number of devices
(limited only by available RAM).  The internal flash is now mounted at
"/flash".

esp8266/modules/_boot.py

@@ -5,7 +5,9 @@ from flashbdev import bdev
 
 try:
     if bdev:
-        vfs = uos.VfsFat(bdev, "")
+        vfs = uos.VfsFat(bdev)
+        uos.mount(vfs, '/flash')
+        uos.chdir('/flash')
 except OSError:
     import inisetup
     vfs = inisetup.setup()