nimble/ll: Add dummy FEM option

Dummy FEM stubs PA and/or LNA APIs so it's possible to test FEM code
with out actualy FEM driver. Only PA/LNA-enable lines are active.

nimble/controller/include/controller/ble_ll_fem.h

@@ -27,16 +27,28 @@ extern "C" {
 #include "syscfg/syscfg.h"
 
 #if MYNEWT_VAL(BLE_LL_FEM_PA)
+#if MYNEWT_VAL(BLE_LL_FEM_DUMMY)
+static inline void ble_ll_fem_pa_init(void) {}
+static inline void ble_ll_fem_pa_enable(void) {}
+static inline void ble_ll_fem_pa_disable(void) {}
+#else
 void ble_ll_fem_pa_init(void);
 void ble_ll_fem_pa_enable(void);
 void ble_ll_fem_pa_disable(void);
 #endif
+#endif
 
 #if MYNEWT_VAL(BLE_LL_FEM_LNA)
+#if MYNEWT_VAL(BLE_LL_FEM_DUMMY)
+static inline void ble_ll_fem_lna_init(void) {}
+static inline void ble_ll_fem_lna_enable(void) {}
+static inline void ble_ll_fem_lna_disable(void) {}
+#else
 void ble_ll_fem_lna_init(void);
 void ble_ll_fem_lna_enable(void);
 void ble_ll_fem_lna_disable(void);
 #endif
+#endif
 
 #if MYNEWT_VAL(BLE_LL_FEM_ANTENNA)
 /* 0 sets default antenna, any other value is FEM specific */

nimble/controller/pkg.yml

@@ -29,9 +29,9 @@ pkg.req_apis:
     - ble_driver
     - ble_transport
     - stats
-pkg.req_apis.BLE_LL_FEM_PA:
+pkg.req_apis.'BLE_LL_FEM_PA && !BLE_LL_FEM_DUMMY':
     - ble_ll_fem_pa
-pkg.req_apis.BLE_LL_FEM_LNA:
+pkg.req_apis.'BLE_LL_FEM_LNA && !BLE_LL_FEM_DUMMY':
     - ble_ll_fem_lna
 pkg.req_apis.BLE_LL_FEM_ANTENNA:
     - ble_ll_fem_antenna

nimble/controller/syscfg.yml

@@ -434,6 +434,11 @@ syscfg.defs:
         description: >
             Enable support for runtime antenna selection in FEM.
         value: 0
+    BLE_LL_FEM_DUMMY:
+        description: >
+            Enable dummy FEM APIs. This enables FEM code but witout using actual
+            FEM driver, i.e. only PA/LNA-enable lines are active.
+        value: 0
 
     BLE_LL_SYSINIT_STAGE:
         description: >