android: Add a script to build OpenSSL's libcrypto as needed by the app

The build script requires the paths to the NDK and OpenSSL sources.

It runs the build in a Docker container, by default. But if the required
tools are installed on the system (currently jq, make and perl) it can
also be run directly on the system by defining NO_DOCKER.

A relatively recent version of the NDK is required (the pre-built
toolchains are required).

src/frontends/android/openssl/compile.sh

@@ -0,0 +1,64 @@
+#!/bin/bash
+#
+# Compile static versions of OpenSSL's libcrypto for use with strongSwan's
+# Android app.
+#
+# Copies archives and header files to $OUT_DIR.
+
+set -e
+
+export PATH=${ANDROID_NDK_ROOT}/toolchains/llvm/prebuilt/linux-x86_64/bin:$PATH
+# necessary for OpenSSL 1.1.1
+export ANDROID_NDK_HOME=${ANDROID_NDK_ROOT}
+
+# automatically determine the ABIs supported by the NDK
+: ${ABIS=$(jq -r 'keys | join(" ")' ${ANDROID_NDK_ROOT}/meta/abis.json)}
+
+# this should match APP_PLATFORM
+: ${MIN_SDK=21}
+
+for ABI in ${ABIS}
+do
+
+echo "## Building OpenSSL's libcrypto for ${ABI}"
+
+case ${ABI} in
+armeabi-v7a)
+	OPTIONS="android-arm"
+	;;
+arm64-v8a)
+	OPTIONS="android-arm64"
+	;;
+x86)
+	OPTIONS="android-x86"
+	;;
+x86_64)
+	OPTIONS="android-x86_64"
+	;;
+esac
+
+OPTIONS="${OPTIONS} \
+  no-shared no-ct no-cast no-comp no-dgram no-dsa no-gost no-idea \
+  no-rmd160 no-seed no-sm2 no-sm3 no-sm4 no-sock no-srp no-srtp \
+  no-asm no-err no-engine no-dso no-hw no-stdio no-ui-console \
+  -fPIC -DOPENSSL_PIC \
+  -ffast-math -O3 -funroll-loops -Wno-macro-redefined \
+  -D__ANDROID_API__=${MIN_SDK} \
+  "
+
+make distclean >/dev/null || true
+
+./Configure ${OPTIONS}
+make -j $(nproc) build_generated >/dev/null
+make -j $(nproc) libcrypto.a >/dev/null
+
+mkdir -p ${OUT_DIR}/${ABI}
+cp libcrypto.a ${OUT_DIR}/${ABI}
+
+done
+
+# The only difference between ABIs is the config header (e.g. configuration.h
+# for OpenSSL 3.0), which does define the size of BN_ULONG in bn.h.
+# However, the only function we use that depends on it is BN_set_word() when
+# generating RSA private keys, which isn't used in the Android app.
+cp -R include/ ${OUT_DIR}