microbit: Added 'bonds' field to Gap::Whitelist_t structure

It is common for application to need to know the number of bonds stored at any
time, for the purposes of bond management. This simple addition allows
applicaitons to discover the number of bonds present using the existing API.

ble/Gap.h

@@ -140,6 +140,7 @@ public:
         BLEProtocol::Address_t *addresses;
         uint8_t                 size;
         uint8_t                 capacity;
+        uint8_t                 bonds;
     };