feat(nix): add android cross-compilation packages

This commit presents the rest of the work to provide android
builds, the packaging and compiling is done trough nix for simplicity

Author: jaoleal

flake.nix

@@ -77,6 +77,81 @@
 						pkgs.gcc.cc.lib
 					];
 				};
+				packages =
+					# Android build infrastructure (unfree NDK + SDK).
+					let
+						androidPkgs = import nixpkgs {
+							inherit system;
+							config.android_sdk.accept_license = true;
+							config.allowUnfree = true;
+						};
+						androidComposition = androidPkgs.androidenv.composeAndroidPackages {
+							platformVersions = [ "34" ];
+							ndkVersions = [ "27.2.12479018" ];
+							includeNDK = true;
+						};
+						androidSdk = androidComposition.androidsdk;
+						androidNdk = "${androidSdk}/libexec/android-sdk/ndk/27.2.12479018";
+
+						mkAndroidPackage =
+							rustTarget:
+							let
+								rustTargetToolchain = fenix.packages.${system}.combine [
+									rustToolchain.rustc
+									rustToolchain.cargo
+									rustToolchain.rust-src
+									rustToolchain.rust-std
+									(fenix.packages.${system}.targets.${rustTarget}.fromToolchainName {
+										name = rustVersion;
+										sha256 = "sha256-ks0nMEGGXKrHnfv4Fku+vhQ7gx76ruv6Ij4fKZR3l78=";
+									}).rust-std
+								];
+								rustPlatform = androidPkgs.makeRustPlatform {
+									cargo = rustTargetToolchain;
+									rustc = rustTargetToolchain;
+								};
+							in
+							rustPlatform.buildRustPackage {
+								pname = "libbitcoinkernel-${rustTarget}";
+								version = "0.2.0";
+								src = ./.;
+								cargoLock.lockFile = ./Cargo-minimal.lock;
+								postPatch = ''
+									cp ${./Cargo-minimal.lock} Cargo.lock
+								'';
+								nativeBuildInputs = [
+									androidPkgs.cmake
+									androidPkgs.boost.dev
+									androidSdk
+								];
+
+								LIBCLANG_PATH = "${androidPkgs.llvmPackages.clang-unwrapped.lib}/lib/";
+								ANDROID_HOME = "${androidSdk}/libexec/android-sdk";
+								ANDROID_NDK_HOME = androidNdk;
+								ANDROID_NDK_ROOT = androidNdk;
+								CMAKE_PREFIX_PATH = "${androidPkgs.boost.dev}";
+
+								# cargoBuildHook hardcodes the host --target at
+								# derivation time, so we bypass it for cross builds.
+								dontCargoBuild = true;
+								doCheck = false;
+								buildPhase = ''
+									cargo build -p libbitcoinkernel-sys --target ${rustTarget} --offline --release
+								'';
+								installPhase = ''
+																				mkdir -p $out/lib $out/include
+																				find target/${rustTarget}/release -path "*/out/install/lib/*.a" \
+																					-exec cp {} $out/lib/ \;
+																				find target/${rustTarget}/release -path "*/out/install/include/*" \
+																					-exec cp {} $out/include/ \;
+																			'';
+							};
+					in
+					{
+						android-aarch64 = mkAndroidPackage "aarch64-linux-android";
+						android-armv7 = mkAndroidPackage "armv7-linux-androideabi";
+						android-x86_64 = mkAndroidPackage "x86_64-linux-android";
+					};
 			}
 		);
 }