Provide ability to build images for other architectures

Adds binaries specified by user to chroot environment
allowing to build  images on amd64,i386,armhf,arm64
architectures.
Closes-Bug: #1332458
Change-Id: If6e63a472ee85559b93b5e6b96d9c3ddcf7bcc98

elements/architecture-emulation-binaries/README.md

@@ -0,0 +1,20 @@
+This element enables execution for different architectures
+
+When building an image for an architecture that the host machine
+can not execute, we need to chroot into the image to execute code,
+and if the host architecture does not match, we need to emulate
+the instructions.
+
+This element does the following:
+
+ * copies the binary file into chroot /usr/bin environment.
+   Binary file is chosen based on host architecture and
+   image architecture the user is trying to build.
+
+   If an image we are building for an architecture is not the host
+   architecture, install tools provided by qemu-user-static
+   (which needs to be installed) to allow us to run commands
+   inside the building image.
+
+   This is tested on amd64/i386 architecture to build armhf and arm64
+   ubuntu cloud images.

elements/architecture-emulation-binaries/cleanup.d/01-cleanbinary

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+set -eux
+set -o pipefail
+
+if [ -x "$TMP_MOUNT_PATH/tmp/clean_up_qemu_binary" ] ; then
+    "$TMP_MOUNT_PATH/tmp/clean_up_qemu_binary"
+    rm -rf "$TMP_MOUNT_PATH/tmp/clean_up_qemu_binary"
+fi

elements/architecture-emulation-binaries/extra-data.d/01-copy-binary

@@ -0,0 +1,61 @@
+#!/bin/bash
+
+set -eux
+set -o pipefail
+
+function clean_up_arch_emulation {
+    cat <<EOF >$TMP_MOUNT_PATH/tmp/clean_up_qemu_binary
+#!/bin/bash
+
+set -eux
+set -o pipefail
+
+sudo rm -rf "${TMP_MOUNT_PATH}${1}"
+EOF
+
+    sudo chmod +x "$TMP_MOUNT_PATH/tmp/clean_up_qemu_binary"
+}
+
+function check_copy_file {
+    if [ -f "$1" ] ; then
+        if [ ! -f "${TMP_MOUNT_PATH}${1}" ]; then
+            sudo cp "$1" "$TMP_MOUNT_PATH/usr/bin"
+            clean_up_arch_emulation $1
+        fi
+    else
+        echo "qemu binary file not found."
+        exit 1
+    fi
+}
+
+function copy_binary {
+    echo "checking for architecture compatibility..."
+    img_arch=$2
+    if [[ "$host_arch" != "$img_arch" ]]; then
+        qemu_binary=$1
+        check_copy_file $qemu_binary
+    else
+        echo "Emulation not required for this host/image architecture combination"
+    fi
+}
+
+host_arch="$(uname -m)"
+
+case "$ARCH" in
+    "i386" | "amd64")
+        qemu_binary_file="/usr/bin/qemu-i386-static"
+        copy_binary $qemu_binary_file $ARCH
+    ;;
+    "armhf")
+        qemu_binary_file="/usr/bin/qemu-arm-static"
+        copy_binary $qemu_binary_file $ARCH
+    ;;
+    "arm64")
+        qemu_binary_file="/usr/bin/qemu-aarch64-static"
+        copy_binary $qemu_binary_file $ARCH
+    ;;
+    *)
+        echo "architecture not supported"
+        exit 1
+    ;;
+esac