apps/blehci: Allow HCI transport to be configured

Even though we always use blehci over UART, it may be useful to have
transport configurable in case someone develops an USB transport or
something else...

apps/blehci/pkg.yml

@@ -17,7 +17,7 @@
 #
 pkg.name: apps/blehci
 pkg.type: app
-pkg.description: BLE controller application exposing HCI over UART
+pkg.description: BLE controller application exposing HCI over external interface
 pkg.author: "Johan Hedberg <johan.hedberg@intel.com>"
 pkg.homepage: "http://mynewt.apache.org/"
 pkg.keywords:
@@ -28,4 +28,7 @@ pkg.deps:
     - "@apache-mynewt-core/sys/stats/full"
     - "@apache-mynewt-core/kernel/os"
     - nimble/controller
-    - nimble/transport/uart
+    - nimble/transport
+
+pkg.req_apis:
+    - ble_transport

apps/blehci/syscfg.yml

@@ -19,3 +19,5 @@
 syscfg.vals:
     # Default task settings
     OS_MAIN_STACK_SIZE: 64
+    # Use UART transport by default
+    BLE_HCI_TRANSPORT: uart