Add a script to build the NDK prebuilts.

Test: OUT_DIR=ndk-out DIST_DIR=ndk-dist \
          ./build/soong/scripts/build-ndk-prebuilts.sh
Bug: None

Change-Id: I3c930d2ec4d73196e355b90766eb14f0f0361460

scripts/build-ndk-prebuilts.sh

@@ -0,0 +1,25 @@
+#!/bin/bash -ex
+
+if [ -z "${OUT_DIR}" ]; then
+    echo Must set OUT_DIR
+    exit 1
+fi
+
+TOP=$(pwd)
+
+SOONG_OUT=${OUT_DIR}/soong
+SOONG_NDK_OUT=${OUT_DIR}/soong/ndk
+rm -rf ${SOONG_OUT}
+mkdir -p ${SOONG_OUT}
+cat > ${SOONG_OUT}/soong.config << EOF
+{
+    "Ndk_abis": true
+}
+EOF
+BUILDDIR=${SOONG_OUT} ./bootstrap.bash
+${SOONG_OUT}/soong ${SOONG_OUT}/ndk.timestamp
+
+if [ -n "${DIST_DIR}" ]; then
+    mkdir -p ${DIST_DIR} || true
+    tar cjf ${DIST_DIR}/ndk_platform.tar.bz2 -C ${SOONG_OUT} ndk
+fi