Restyle Adding the steps to build CHIP for Android apps.

src/android/CHIPTool/README.md

@@ -6,3 +6,89 @@ a CHIP device
 
 -   Connection to the chip device's soft AP will be a manual step until pairing
     is implemented.
+
+===========================================
+
+Steps to build CHIP for Android apps
+
+Pre-conditions: Have Android NDK downloaded to your machine. Set
+\$ANDROID_NDK_HOME environment variable to point to the NDK package is
+downloaded.
+
+1. Checkout the CHIP repo
+
+2. In commandline / Terminal, 'cd' into the top CHIP directory
+
+3. Run './bootstrap'
+
+4. Exit out of the checked out CHIP repo directory and using 'mkdir [some-dir]'
+   create a new empty one where you want the generated output files to go.
+
+5. 'cd' into this newly created directory
+
+6. Export the following variables
+
+ABIs that can be used as TARGET:
+
+| ABI         | Triple                   |
+| ----------- | ------------------------ |
+| armeabi-v7a | armv7a-linux-androideabi |
+| arm64-v8a   | aarch64-linux-android    |
+| x86         | i686-linux-android       |
+| x86-64      | x86_64-linux-android     |
+
+NDK OS Variants:
+
+| NDK OS Variant | Host Tag       |
+| -------------- | -------------- |
+| macOS          | darwin-x86_64  |
+| Linux          | linux-x86_64   |
+| 32-bit Windows | windows        |
+| 64-bit Windows | windows-x86_64 |
+
+```
+OS_VARIANT=darwin-x86_64 // Specify OS variant corresponding to system
+export TOOLCHAIN=$ANDROID_NDK_HOME/toolchains/llvm/prebuilt/$OS_VARIANT
+export TARGET=aarch64-linux-android // Use desired ABI
+export API=21
+export AR=$TOOLCHAIN/bin/$TARGET-ar
+export AS=$TOOLCHAIN/bin/$TARGET-as
+export CC=$TOOLCHAIN/bin/$TARGET$API-clang
+export CXX=$TOOLCHAIN/bin/$TARGET$API-clang++
+export LD=$TOOLCHAIN/bin/$TARGET-ld
+export RANLIB=$TOOLCHAIN/bin/$TARGET-ranlib
+export STRIP=$TOOLCHAIN/bin/$TARGET-strip
+export JNI_INCLUDE_DIRS=$ANDROID_NDK_HOME/sysroot/usr/include
+```
+
+Note: Regardless of where JAVA_HOME points, always use the JNI headers from the
+Android NDK, and only include the top-most directory (include), not the system
+directory (include/linux). Because the NDK mixes the JNI headers in with the
+linux headers, listing the system directory in the -I flags will result in
+strange compilation errors. And unlike the standard Java jni.h, the jni.h that
+comes with the Android NDK does not depend on any system-specific JNI header
+files (e.g. jni_md.h). Thus only the top-level include directory is needed.
+
+7. Then run '../connectedhomeip/configure --host=\$TARGET --with-crypto=mbedtls
+   --enable-tests=no --enable-shared --with-device-layer=none'
+
+8. Finally 'make'
+
+9. You should see the generated SetupPayloadParser.jar under
+   src/setup_payload/java and libSetupPayloadParser.so under
+   src/setup_payload/java/.libs folders in the output directory.
+
+Drop the .jar in the libs folder in the Android project.
+
+Drop the .so in the ABI-specific jniLibs folder in the Android project, eg.
+jniLibs/arm64-v8a
+
+'Gradle sync' the Android project and run.
+
+10. You will also need the "libc++\_shared.so" file in the jniLibs folder. This
+    file comes packaged with Android NDK and can be found under
+    $ANDROID_NDK_HOME/sources/cxx-stl/llvm-libc++/libs/$TARGET
+
+(Eventually hoping to not have to include this .so, but that needs some more
+tweaking of the Android automake build rules. Include it in the interim to be
+able to build the Android app).