host: Add defines for Key Distribution settings

Add key distribution masks defines. This can be used to initialize sm_our_key_dist
and sm_their_key_dist fields in ble_hs_cfg struct.

nimble/host/include/host/ble_hs.h

@@ -165,6 +165,25 @@ extern "C" {
 /** KeyboardDisplay Only IO capability */
 #define BLE_HS_IO_KEYBOARD_DISPLAY          0x04
 
+/**
+ * @}
+ */
+
+/**
+ * @brief LE key distribution
+ * @defgroup bt_host_key_dist LE key distribution
+ *
+ * @{
+ */
+
+/** Distibute LTK */
+#define BLE_HS_KEY_DIST_ENC_KEY              0x01
+
+/** Distribute IRK */
+#define BLE_HS_KEY_DIST_ID_KEY               0x02
+
+/** CSRK distibution and LinkKey are not supported */
+
 /**
  * @}
  */