Update btstack

This is needed to be compatible with "new" pico-sdk versions.
See raspberrypi/pico-sdk#1553 and #1

Unfortunately this will break compatibility with older pico-sdk
versions.

README.md

@@ -29,3 +29,25 @@ make
 ```
 
 The `.uf2` file will be `build/src/picow_ds4.uf2`
+
+# Known Issues
+
+`pico-sdk` implements its own `btstack` makefile (see
+`$PICO_SDK_PATH/src/rp2_common/pico_btstack/CMakeLists.txt`), which means a
+particular `pico-sdk` version is only technically compatible with a single
+`btstack` version. See https://github.com/raspberrypi/pico-sdk/issues/1553 and
+#1.
+
+If you run in to build issues which look related to `btstack`, it could be
+because the version this project depends on has diverged from your version
+of `pico-sdk` again.
+
+This repo is known to build OK against `pico-sdk` version
+`6a7db34ff63345a7badec79ebea3aaef1712f374` (tag: 1.5.1).
+
+There's two good ways to fix this, which maybe I'll do one day:
+
+1. Upstream a configurable `MAX_ATTRIBUTE_VALUE_SIZE` to `btstack`, so that the
+   `btstack` submodule could be removed here.
+2. Change `btstack`+`pico-sdk` to put the make rules inside `btstack` instead,
+   so that there's no implicit dependency between the two.