Include btstack.h instead of uni.h; note the include-path prerequisite

Nothing in the sketch calls a uni_ function - bd_addr_t, sscanf_bd_addr,
gap_connect and the gatt_client calls are all BTstack.

ps4_lego_onebrain/ps4_lego_onebrain.ino

@@ -5,10 +5,16 @@
  * LEGO Powered Up hubs over BLE, driven by a hand-rolled LWP3 GATT client that
  * runs on the same BTstack instance.
  *
- * Board package: esp32_bluepad32  (NOT the plain esp32 package)
+ * Board package: esp32-bluepad32 4.1.0  (NOT the plain esp32 package)
  * Libraries:     none. No Legoino, no NimBLE - they would be a second host
  *                stack and that is exactly what we are avoiding.
  *
+ * BEFORE THIS WILL COMPILE:
+ *   The board package ships the BTstack headers but does not put them on the
+ *   include path. You must add a platform.local.txt to the package folder
+ *   first - see docs/BUILD.md. Without it you get:
+ *     fatal error: btstack.h: No such file or directory
+ *
  * THREADING - read this before changing anything:
  *   BTstack is not thread-safe. loop() runs in the Arduino task, BTstack runs
  *   in its own task. The only BTstack call allowed from loop() is
@@ -26,7 +32,7 @@
  */
 
 #include <Bluepad32.h>
-#include <uni.h>
+#include <btstack.h>
 
 // =========================================================== configuration