switch between internal and external sdcard

frontend/device/generic/device.lua

@@ -45,6 +45,7 @@ local Device = {
     needsTouchScreenProbe = no,
     hasClipboard = yes, -- generic internal clipboard on all devices
     hasEinkScreen = yes,
+    hasExternalSD = no, -- or other storage volume that cannot be accessed using the File Manager
     canHWDither = no,
     canHWInvert = no,
     canUseCBB = yes, -- The C BB maintains a 1:1 feature parity with the Lua BB, except that is has NO support for BB4, and limited support for BBRGB24