Merge #16110: depends: Add Android NDK support

f9af3ced1c Android: add all arch support (Block Mechanic)
d419ca7e32 depends: export dynamic JNI symbols from static qtforandroid.a (Igor Cota)
ed30684d03 Qt: patch androidjnimain.cpp to make sure JNI is initialised when statically compiled (Igor Cota)
e4c319e8a1 builds: remove superfluous config_opts_aarch64_android (Igor Cota)
24ffef0c27 Patch libevent when building for Android (fix arc4random_addrandom) (Igor Cota)
f1e40b3e71 Update bitcoin_qt.m4 (BlockMechanic)
b4057d8261 Define TARGET_OS when host is android (Igor Cota)
80b475f159 Fix Android zlib cross compilation issue (https://stackoverflow.com/questions/21396988/zlib-build-not-configuring-properly-with-cross-compiler-ignores-ar) (Igor Cota)
45f8219015 Add full Android build example command and instructions on getting SDK/NDK (Igor Cota)
b68f2a68c2 Add config opts and patch for aarch64_android build of Qt (Igor Cota)
9c4cb0166e Add ranlib to android.mk hosts file (fix OSX Android NDK build) (Igor Cota)
c2a749c9c1 Add example Android host-platform-triplet and options (Igor Cota)
0b0cff3c61 Add support for building Android dependencies (Igor Cota)

Pull request description:

  This allows one to build the dependencies with the Android SDK and goes towards fixing #11844. It has been tested to work with:
  `make HOST=aarch64-linux-android ANDROID_API_LEVEL=28 ANDROID_TOOLCHAIN_BIN=/home/user/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/bin NO_QT=1 NO_WALLET=1`

ACKs for top commit:
  Sjors:
    ACK f9af3ce. I'm OK with merging and then improving later.

Tree-SHA512: cb805115ebe5c9e33db2bf3eab8628808fe3f50052053d8877d8b8e4406d6fea1ed9e5c4dff85d777fb99c81be6ffb9d95a0e6d32344e728e5e0da6c653e2ce7

depends/README.md

@@ -30,8 +30,17 @@ Common `host-platform-triplets` for cross compilation are:
 - `aarch64-linux-gnu` for Linux ARM 64 bit
 - `riscv32-linux-gnu` for Linux RISC-V 32 bit
 - `riscv64-linux-gnu` for Linux RISC-V 64 bit
+- `aarch64-linux-android` for Android ARM 64 bit
+
+The paths are automatically configured and no other options are needed unless targeting Android.
+Before proceeding with an Android build one needs to get the [Android SDK](https://developer.android.com/studio) and use the "SDK Manager" tool to download the NDK and one or more "Platform packages" (these are Android versions and have a corresponding API level).
+In order to build `ANDROID_API_LEVEL` (API level corresponding to the Android version targeted, e.g. Android 9.0 Pie is 28 and its "Platform package" needs to be available) and `ANDROID_TOOLCHAIN_BIN` (path to toolchain binaries depending on the platform the build is being performed on) need to be set.
+If the build includes Qt, environment variables `ANDROID_SDK` and `ANDROID_NDK` need to be set as well but can otherwise be omitted.
+This is an example command for a default build with no disabled dependencies:
+
+    ANDROID_SDK=/home/user/Android/Sdk ANDROID_NDK=/home/user/Android/Sdk/ndk-bundle make HOST=aarch64-linux-android ANDROID_API_LEVEL=28 ANDROID_TOOLCHAIN_BIN=/home/user/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/bin
+
 
-No other options are needed, the paths are automatically configured.
 
 ### Install the required dependencies: Ubuntu & Debian