usb: Add ability to prompt user about what to do upon usb insertion

v3: Add in config option
 v4: Bugfixes
 v5: Force a redraw upon exiting
 v6: keypress-in-chargeonly mode enables mass storage (and vice versa)
 v7: Fix bootloader builds
 v8: Update manual, and have bootloader respect keypresses
 v9: Change default to mass storage (ie no change in behavior)

 todo:
  * test-build dx50/dx90
  * Switch from yes/no to proper menu?
  * prevent WPS progress bar from drawing over us

Change-Id: I82e0ccb08497b7a5aa756ce77f1332ee963703a7

...

Change-Id: I7946cf240b18a4fa8ace5e25e1eb6e97b8b12d7c

firmware/export/usb.h

@@ -131,6 +131,15 @@ enum
 #endif
 };
 
+/* Supported usb modes. */
+enum
+{
+    USB_MODE_ASK = 0,
+    USB_MODE_MASS_STORAGE,
+    USB_MODE_CHARGE,
+    USB_MODE_ADB
+};
+
 #ifdef HAVE_USB_POWER
 /*allow people to define this in config-target.h if they need it*/
 #if !defined(USBPOWER_BTN_IGNORE)
@@ -215,6 +224,7 @@ void usb_charging_maxcurrent_change(int maxcurrent);
 /* returns the maximum allowed USB current, based on USB charging mode and state */
 int usb_charging_maxcurrent(void);
 #endif /* HAVE_USB_CHARGING_ENABLE */
+void usb_set_mode(int mode);
 #endif /* HAVE_USB_POWER */
 #ifdef HAVE_USBSTACK
 /* USB driver call this function to notify that a transfer has completed */