[doc] Move runtime android build guide (#12456)

This commit moves android build guide documentation from nnfw/howto/ to howto.

ONE-DCO-1.0-Signed-off-by: Hyeongseok Oh <[email protected]>

docs/howto/how-to-cross-build-runtime-for-android.md

@@ -1,2 +1,41 @@
 # How to Cross-build Runtime for Android
 
+**Note: To set up a basic build environment on your development PC, please read the [how-to-build-runtime.md](how-to-build-runtime.md) document first. The cross build covered in this document assumes that you have an environment in which the native build operates normally without problems.**
+
+Supported Architecture : AARCH64 only (ARM32 is not supported yet)
+
+## Prepare Android NDK
+
+Use `tools/cross/install_android_ndk.sh` script to prepare Android NDK. This is recommended way to build Android NDK.
+
+Or you can use `tools/cross/install_android_sdk.sh` script to prepare Android SDK including NDK. You can find NDK in `{android-sdk-dir}/ndk/{ndk-version}` directory.
+
+## Build
+
+### Host Environment Requirements
+
+CMake 3.6.0 or later is required for Android NDK r20 CMake support.
+So if you want to use Docker, please use `infra/docker/focal/Dockerfile` which is based on Ubuntu 20.04. It has CMake 3.16.3.
+
+```
+$ ./nnas build-docker-image -t nnfw/one-devtools:focal
+```
+
+
+### Build and install the runtime
+
+Some tools/libs are still not supported and those are not built by default - mostly due to dependency on HDF5 library.
+Please refer to `infra/nnfw/cmake/options/options_aarch64-android.cmake` for details.
+
+Different from cross build for linux,
+
+- `NDK_DIR` is required
+
+Here is an example of using Makefile.
+
+```bash
+TARGET_OS=android \
+CROSS_BUILD=1 \
+NDK_DIR=/path/android-sdk/ndk/{ndk-version}/ \
+make -f Makefile.template install
+```