Make advertisement intervals configurable

We were previously relying on the default values (100ms, 150ms). This
PR makes it easier to test and configure parameters for different
tradeoffs between range / power consumption (#98).

code/nrf-connect/samples/ble/Kconfig

@@ -10,6 +10,14 @@ config PRST_BLE_ADV_DURATION_SEC
 	int "Advertising duration in seconds"
 	default 1
 
+config PRST_BLE_MIN_ADV_INTERVAL
+	int "Minimum advertising interval in milliseconds"
+	default 100
+
+config PRST_BLE_MAX_ADV_INTERVAL
+	int "Maximum advertising interval in milliseconds"
+	default 150
+
 choice PRST_BLE_ENCODING
 	prompt "b-parasite BLE encoding"
 	default PRST_BLE_ENCODING_BPARASITE_V2

code/nrf-connect/samples/ble/src/ble.c

@@ -55,11 +55,19 @@ int prst_ble_init() {
   return 0;
 }
 
+#define PRST_MS_TO_INTERVAL(value_ms) ((uint16_t)(value_ms) / 0.625f)
+
 int prst_ble_adv_start() {
   // If BT_LE_ADV_NCONN_IDENTITY, this function will advertise with a static MAC
   // address programmed in the chip. If BT_LE_ADV_NCONN, this function returns
   // advertises with a random MAC each time.
-  return bt_le_adv_start(BT_LE_ADV_NCONN_IDENTITY, ad, ARRAY_SIZE(ad), sd,
+  return bt_le_adv_start(
+      BT_LE_ADV_PARAM(
+          BT_LE_ADV_OPT_USE_IDENTITY,
+          PRST_MS_TO_INTERVAL(CONFIG_PRST_BLE_MIN_ADV_INTERVAL),
+          PRST_MS_TO_INTERVAL(CONFIG_PRST_BLE_MAX_ADV_INTERVAL),
+          NULL),
+      ad, ARRAY_SIZE(ad), sd,
       ARRAY_SIZE(sd));
 }