NRF51822: Fix baremetal linker error

Ensure the NRF51822 `us_ticker.c` module content is compiled in only if
the target supports USTICKER.

targets/TARGET_NORDIC/TARGET_MCU_NRF51822/us_ticker.c

@@ -13,6 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+#if DEVICE_USTICKER
 #include <stddef.h>
 #include <stdbool.h>
 #include "us_ticker_api.h"
@@ -613,3 +615,4 @@ uint32_t os_tick_val(void) {
         return clock_cycles_by_tick - ((current_counter - next_tick_cc_value) % clock_cycles_by_tick);
     }
 }
+#endif // DEVICE_USTICKER

targets/targets.json

@@ -5043,8 +5043,13 @@
             "SERIAL",
             "SLEEP",
             "SPI",
-            "SPISLAVE"
-        ]
+            "SPISLAVE",
+            "USTICKER"
+        ],
+        "overrides": {
+            "tickless-from-us-ticker"   : true,
+            "boot-stack-size"           : "0x400"
+        }
     },
     "MCU_NRF51_16K_BASE": {
         "inherits": ["MCU_NRF51"],