Improve (Open-)Harmony OS build instructions (#31)

* Improve (Open-)Harmony OS build instructions

Clarify how to build for HarmonyOS, improve the documentation
of the environment varibales and mention `direnv` as a convenient
tool to automatically set the environment variables.

Signed-off-by: Jonathan Schwender <[email protected]>

* Minor edits for clarity

* Fix missing verb

Signed-off-by: Jonathan Schwender <[email protected]>

---------

Signed-off-by: Jonathan Schwender <[email protected]>
Co-authored-by: Delan Azabani <[email protected]>

src/hacking/building-for-openharmony.md

@@ -155,33 +155,54 @@ The value of the variable must be a file path to a valid `.json` file with the s
 
 ## Building servoshell
 
-Before building servo, ensure the following environment variables are set in the current shell:
+Before building servo you will need to set some environment variables.
+[direnv](https://direnv.net) is a convenient tool that can automatically set these variables based on an `.envrc` file, but you can also use any other method to set the required environment variables.
 
+`.envrc`:
 ```commandline
+    export OHOS_SDK_NATIVE=/path/to/openharmony-sdk/platform/api-version/native
+
+    # Required if the HAP must be signed. See the signing configuration section above.
+    export SERVO_OHOS_SIGNING_CONFIG=/path/to/signing-configs.json
+
+    # Required only when building for HarmonyOS:
+    export DEVECO_SDK_HOME=/path/to/command-line-tools/sdk # OR /path/to/DevEcoStudio/sdk OR on MacOS /Applications/DevEco-Studio.app/Contents/sdk
+
+    # Required only when building for OpenHarmony:
     # Note: The openharmony sdk is under ${DEVECO_SDK_HOME}/default/openharmony
     # Presumably you would need to replicate this directory structure
-    export OHOS_SDK_NATIVE=/path/to/openharmony-sdk/platform/api-version/native
     export OHOS_BASE_SDK_HOME=/path/to/openharmony-sdk/platform
-    export HVIGOR_PATH=/path/to/parent/directory/containing/node_modules # not required if hvigorw is in PATH
-    export SERVO_OHOS_SIGNING_CONFIG=/path/to/signing-configs.json # required if the HAP must be signed.
+
+    # If you have the command-line tools installed:
+    export PATH="${PATH}:/path/to/command-line-tools/bin/"
+    export NODE_HOME=/path/to/command-line-tools/tool/node
+
+    # Alternatively, if you do NOT have the command-line tools installed:
+    export HVIGOR_PATH=/path/to/parent/directory/containing/node_modules  # Not required if `hvigorw` is in $PATH
 ```
 
+If you use `direnv` and an `.envrc` file, please remember to run `direnv allow .` after modifying the `.envrc` file.
+Otherwise, the environment variables will not be loaded.
+
 The following command can then be used to compile the servoshell application for a 64-bit ARM device or emulator:
 
 ```commandline
-./mach build --ohos --release
+./mach build --ohos --release [--flavor=harmonyos]
 ```
 
 In `mach build`, `mach install` and `mach package` commands, `--ohos` is an alias for `--target aarch64-unknown-linux-ohos`.
-To build for an emulator running on an x86-64 host, use `--target x86_64-unknown-linux-ohos`
+To build for an emulator running on an x86-64 host, use `--target x86_64-unknown-linux-ohos`.
+The default `ohos` build / package / install targets OpenHarmony.
+If you want to build for HarmonyOS you can add `--flavor=harmonyos`.
+Please check the [Signing configuration](#signing-configuration) and add a configuration with `"name": "hos"` and `"type": "HarmonyOS""` and the respective signing certificates.
 
 
 ## Installing and running on-device
 
 The following command can be used to install previously built servoshell application on a 64-bit ARM device or emulator:
 
 ```commandline
-./mach install --ohos --release
+./mach install --ohos --release [--flavor=harmonyos]
 ```
 
 ## Further reading