iBasso DX50/DX90: User selectable USB mode.

Depends on http://gerrit.rockbox.org/r/#/c/1043/.

This patch adds a new setting in Settings -> General -> System: USB Mode.

Usable in Quickscreen and Shortcuts.

Possible settings are:
- Mass Storage: The default, on USB connect export the internal and external
drives as usual. Rockbox will exit gracefully in this case, since the internal
drive (/mnt/sdcard) is exported as mass storage device and Android prevents
Rockbox from continued execution.
- Charge Only: USB charge only, do not export the internal and external drives.
This will not close Rockbox.
- ADB: Enable the Android Debug Bridge. This will not close Rockbox.

Charge only and ADB are actually the same, since it is not yet established if
charge only is doable without adb and major hooks into Android.

German translation provided.

This may be genric for Android based devices but is only enabled
for iBasso Devices. Other maintainers may choose do adopt this.

Change-Id: I616247c29946c6595ebcf8f0c14b2410c9f0498b

firmware/target/hosted/ibasso/system-ibasso.c

@@ -32,7 +32,6 @@
 #include "button-ibasso.h"
 #include "debug-ibasso.h"
 #include "sysfs-ibasso.h"
-#include "usb-ibasso.h"
 #include "vold-ibasso.h"
 
 
@@ -50,7 +49,6 @@ void system_init(void)
     stackbegin = stackend = (uintptr_t*) &stack;
 
 	vold_monitor_start();
-    ibasso_set_usb_mode(USB_MODE_MASS_STORAGE);
 
     /*
         Prevent device from deep sleeping, which will interrupt playback.