esp8266/scripts/_boot: Mount block device on "" instead of "/".

"" is the correct name of the root directory when mounting a device there
(as opposed to "/").  One can now do os.listdir('/') and open('/abc'), as
well as os.listdir() and open('abc').

esp8266/scripts/_boot.py

@@ -3,10 +3,10 @@ import builtins
 from flashbdev import bdev
 
 try:
-    vfs = uos.VfsFat(bdev, "/")
+    vfs = uos.VfsFat(bdev, "")
 except OSError:
     import inisetup
     inisetup.check_bootsec()
     uos.VfsFat.mkfs(bdev)
-    vfs = uos.VfsFat(bdev, "/")
+    vfs = uos.VfsFat(bdev, "")
     inisetup.wifi()