Add occlum busybox prebuild script into toolchain

Signed-off-by: Zheng, Qi <huaiqing.zq@antgroup.com>

tools/toolchains/busybox/build.sh

@@ -0,0 +1,45 @@
+#! /bin/bash
+set -e
+THIS_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}"  )" >/dev/null 2>&1 && pwd )"
+INSTALL_DIR=/opt/occlum/toolchains/busybox
+
+busybox_source="https://github.com/mirror/busybox.git"
+busybox_tag="1_31_1"
+busybox_config=${THIS_DIR}/config
+
+function build_and_install_busybox()
+{
+    pushd busybox
+    git clean -dxf
+    git reset HEAD --hard
+    make clean
+    make defconfig
+    cp $busybox_config .config
+
+    if [[ $1 == "musl" ]]; then
+        echo "Building musl-libc version of busybox"
+        sed -e 's/.*CONFIG_CROSS_COMPILER_PREFIX.*/CONFIG_CROSS_COMPILER_PREFIX="occlum-"/' -i .config
+        make -j
+        mkdir -p ${INSTALL_DIR}/musl
+        cp busybox ${INSTALL_DIR}/musl/
+    else
+        echo "Building glibc version of busybox"
+        make -j
+        mkdir -p ${INSTALL_DIR}/glibc
+        cp busybox ${INSTALL_DIR}/glibc/
+    fi
+
+    popd
+}
+
+rm -rf ${INSTALL_DIR}
+rm -rf busybox
+# Download bash source
+git clone -b ${busybox_tag} ${busybox_source}
+
+echo "Building busybox with musl-gcc (occlum-gcc) ..."
+build_and_install_busybox musl
+
+echo "Building busybox with gcc ..."
+build_and_install_busybox
+