libbinder: update Trusty README

# Binder for Trusty

This is the Trusty port of the libbinder library.

To build it, take the following steps:

* Check out copies of the Trusty and AOSP repositories.

* Apply the patches from the `trusty_binder` topic on both repositories.

* Build Trusty normally using `build.py`.

* Run the sample AIDL test for Trusty:

To build it, first you will need a checkout of the Trusty tree:

```shell

  $ ./build-root/.../run --headless --boot-test com.android.trusty.aidl.test

$ mkdir /path/to/trusty

$ cd /path/to/trusty

$ repo init -u https://android.googlesource.com/trusty/manifest -b master

$ repo sync -j$(nproc) -c --no-tags

```

To run the Android-Trusty IPC test, do the following:

* Build AOSP for the `qemu_trusty_arm64-userdebug` target:

After the checkout is complete, you can use the `build.py` script for both

building and testing Trusty. For a quick build without any tests, run:

```shell

  $ lunch qemu_trusty_arm64-userdebug

  $ m

$ ./trusty/vendor/google/aosp/scripts/build.py generic-arm64-test-debug

```

* In the Trusty directory, run the emulator with the newly built Android:

This will build the smaller `generic-arm64-test-debug` project which

does not run any tests.

The qemu-generic-arm64-test-debug` project includes the QEMU emulator and

a full Trusty test suite, including a set of libbinder tests.

To run the latter, use the command:

```shell

  $ ./build-root/.../run --android /path/to/aosp

$ ./trusty/vendor/google/aosp/scripts/build.py \

    --test "boot-test:com.android.trusty.binder.test" \

    qemu-generic-arm64-test-debug

```

* Using either `adb` or the shell inside the emulator itself, run the Trusty

  Binder test as root:

  ```shell

  # /data/nativetest64/vendor/trusty_binder_test/trusty_binder_test

## Building AIDL files on Trusty

To compile AIDL interfaces into Trusty libraries, include the `make/aidl.mk`

in your `rules.mk` file, e.g.:

```

LOCAL_DIR := $(GET_LOCAL_DIR)

## Running the AIDL compiler

For now, you will need to run the AIDL compiler manually to generate the C++

source code for Trusty clients and services. The general syntax is:

```shell

$ aidl --lang=cpp -o <output directory> -h <output header directory> <AIDL files...>

MODULE := $(LOCAL_DIR)

MODULE_AIDLS := \

        $(LOCAL_DIR)/IFoo.aidl \

include make/aidl.mk

```

The compiler will emit some `.cpp` files in the output directory and their

corresponding `.h` files in the header directory.

## Examples

The Trusty tree contains some sample test apps at

`trusty/user/app/sample/binder-test`.