Improve the README.

README.md

@@ -1,19 +1,45 @@
 # Template repository for creating a custom Toit envelope
 
+This repository can be used to create custom [envelopes](https://docs.toit.io/tutorials/containers)
+for [Toit](https://toitlang.org/).
+
+There are already many [pre-built envelopes](https://github.com/toitlang/envelopes) available, but
+if you need to create a custom envelope, this repository can be used as a starting point.
+
+## External services
+
+See the [README-external-service.md](README-external-service.md) file for information on how to use external services.
+
 ## Setup
 
-* Fork this repository (giving it a better name), and then detach the fork: https://support.github.com/request/fork
-* Change the license to your license.
-* Change the `TARGET` variable in the Makefile to the name of your chip. By default it is set to `esp32`.
-* Run `make init`. This will copy some of the Toit files, depending on the target, to your repository.
+### Prerequisites
 * Make sure you have a complete build environment. See
   - https://github.com/toitlang/toit, and
   - https://docs.espressif.com/projects/esp-idf/en/latest/esp32/get-started/index.html
   - A good starting point is to run `install.sh` from the `toit/third_party/esp-idf` folder.
 
+The [ci.yml](.github/workflows/ci.yml) file uses Toit's setup action to install all prerequisites
+on a GitHub runner. You can use this as a reference for setting up your own environment.
 
-## Makefile targets
+### Initial setup
 
+* Duplicate this repository, following
+  [GitHub's instructions](https://docs.github.com/en/repositories/creating-and-managing-repositories/duplicating-a-repository).
+  If you forked it, you can also detach the fork: https://support.github.com/request/fork
+* Change the license to your license.
+* Change the `TARGET` variable in the Makefile to the name of your chip. By default it is set to `esp32`.
+* Run `make init`. This will copy some of the Toit files, depending on the target, to your repository.
+
+### Configuration
+* Adjust or remove the C components in the `components` folder.
+* Run `make menuconfig` to configure the build.
+* Adjust the [ci.yml](.github/workflows/ci.yml) file to match your setup. Typically, you don't need
+  to compile on Windows or macOS.
+
+### Build
+* Run `make` to build the envelope. It should end up with a `build/esp32/firmware.envelope`.
+
+## Makefile targets
 - `make` or `make all` - Build the envelope.
 - `make init` - Initialize after cloning. See the Setup section above.
 - `make menuconfig` - Runs the ESP-IDF menuconfig tool in the build-root. Also creates the `sdkconfig.defaults` file.

components/echo/echo.c

@@ -36,7 +36,7 @@ static toit_err_t on_created(void* user_data, toit_msg_context_t* context) {
 /// @param data The data of the message.
 /// @param length The length of the message.
 /// @return TOIT_ERR_SUCCESS. The only allowed return value.
-static toit_err_t on_message(void* user_data, int sender, void* data, int length) {
+static toit_err_t on_message(void* user_data, int sender, uint8_t* data, int length) {
   echo_service_t* echo_service = (echo_service_t*)user_data;
   toit_msg_context_t* context = echo_service->msg_context;
   if (toit_msg_notify(context, sender, data, length, true) != TOIT_ERR_SUCCESS) {
@@ -58,7 +58,7 @@ static toit_err_t on_message(void* user_data, int sender, void* data, int length
 /// @param data The data of the request.
 /// @param length The length of the request.
 /// @return TOIT_ERR_SUCCESS. The only allowed return value.
-static toit_err_t on_rpc_request(void* user_data, int sender, int function, toit_msg_request_handle_t handle, void* data, int length) {
+static toit_err_t on_rpc_request(void* user_data, int sender, int function, toit_msg_request_handle_t handle, uint8_t* data, int length) {
   if (toit_msg_request_reply(handle, data, length, true) != TOIT_ERR_SUCCESS) {
     printf("unable to reply\n");
   }
@@ -81,10 +81,10 @@ static void __attribute__((constructor)) init() {
   echo_service_t* echo_service = (echo_service_t*)malloc(sizeof(echo_service_t));
   echo_service->msg_context = NULL;
   toit_msg_cbs_t cbs = {
-    .on_created = &on_created,
-    .on_message = &on_message,
-    .on_rpc_request = &on_rpc_request,
-    .on_removed = &on_removed,
+    .on_created = on_created,
+    .on_message = on_message,
+    .on_rpc_request = on_rpc_request,
+    .on_removed = on_removed,
   };
   toit_msg_add_handler("toitlang.org/demo-echo", echo_service, cbs);
 }