Add callback functionality for thumb drive disconnects

Adds callback functionality for thumb drive disconnects that works similarly to the connect
callback functionality.

Author: Alrik Vidstrom

src/USBHost/USBHost.cpp

@@ -31,6 +31,7 @@ USBHost * USBHost::instHost = NULL;
 #define MIN(a, b) ((a > b) ? b : a)
 
 extern void (*mount_fnc)(void);
+extern void (*unmount_fnc)(void);
 
 /**
 * How interrupts are processed:
@@ -256,6 +257,10 @@ void USBHost::usb_process()
 
                     } while(0);
 
+                    if (nullptr != unmount_fnc) {
+                        unmount_fnc();
+                    }
+
                     break;
 
                 // a td has been processed

src/USBHostMSD/USBHostMSD.cpp

@@ -29,6 +29,7 @@
 #define BO_MASS_STORAGE_RESET   (0xFF)
 
 void (*mount_fnc)(void) = nullptr;
+void (*unmount_fnc)(void) = nullptr;
 
 USBHostMSD::USBHostMSD()
 {
@@ -442,4 +443,9 @@ bool USBHostMSD::attach_detected_callback(void (*cbk)()) {
     return true;
 }
 
+bool USBHostMSD::attach_removed_callback(void (*cbk)()) {
+    unmount_fnc = cbk;
+    return true;
+}
+
 #endif

src/USBHostMSD/USBHostMSD.h

@@ -65,7 +65,7 @@ class USBHostMSD : public IUSBEnumerator, public mbed::BlockDevice
     virtual mbed::bd_size_t size() const;
     virtual const char *get_type() const;
     bool attach_detected_callback(void (*cbk)());
-
+    bool attach_removed_callback(void (*cbk)());
 
 protected:
     //From IUSBEnumerator