scripts: add helper script get-flatcar

Similar to `get-coreos`, add a helper script `get-flatcar` to download
Flatcar assets.

Follow up for poseidon/typhoon#315

scripts/get-flatcar

@@ -0,0 +1,83 @@
+#!/usr/bin/env bash
+# USAGE: ./scripts/get-flatcar
+# USAGE: ./scripts/get-flatcar channel version dest
+#
+# ENV VARS:
+# - OEM_ID - specify OEM image id to download, alongside the default one
+set -eou pipefail
+
+GPG=${GPG:-/usr/bin/gpg}
+
+CHANNEL=${1:-"stable"}
+VERSION=${2:-"current"}
+DEST_DIR=${3:-"$PWD/examples/assets"}
+OEM_ID=${OEM_ID:-""}
+DEST=$DEST_DIR/flatcar/$VERSION
+BASE_URL=https://$CHANNEL.release.flatcar-linux.net/amd64-usr/$VERSION
+
+# check channel/version exist based on the header response
+if ! curl -s -I "${BASE_URL}/flatcar_production_pxe.vmlinuz" | grep -q -E '^HTTP/[0-9.]+ [23][0-9][0-9]'; then
+  echo "Channel or Version not found"
+  exit 1
+fi
+
+if [[ ! -d "$DEST" ]]; then
+  echo "Creating directory ${DEST}"
+  mkdir -p "${DEST}"
+fi
+
+if [[ -n "${OEM_ID}" ]]; then
+  IMAGE_NAME="flatcar_production_${OEM_ID}_image.bin.bz2"
+
+  # check if the oem version exists based on the header response
+  if ! curl -s -I "${BASE_URL}/${IMAGE_NAME}" | grep -q -E '^HTTP/[0-9.]+ [23][0-9][0-9]'; then
+    echo "OEM version not found"
+    exit 1
+  fi
+fi
+
+echo "Downloading Flatcar Linux $CHANNEL $VERSION images and sigs to $DEST"
+
+echo "Flatcar Linux Image Signing Key"
+curl -# https://www.flatcar-linux.org/security/image-signing-key/Flatcar_Image_Signing_Key.asc -o "${DEST}/Flatcar_Image_Signing_Key.asc"
+$GPG --import <"$DEST/Flatcar_Image_Signing_Key.asc" || true
+
+# Version
+echo "version.txt"
+curl -# "${BASE_URL}/version.txt" -o "${DEST}/version.txt"
+
+# PXE kernel and sig
+echo "flatcar_production_pxe.vmlinuz..."
+curl -# "${BASE_URL}/flatcar_production_pxe.vmlinuz" -o "${DEST}/flatcar_production_pxe.vmlinuz"
+echo "flatcar_production_pxe.vmlinuz.sig"
+curl -# "${BASE_URL}/flatcar_production_pxe.vmlinuz.sig" -o "${DEST}/flatcar_production_pxe.vmlinuz.sig"
+
+# PXE initrd and sig
+echo "flatcar_production_pxe_image.cpio.gz"
+curl -# "${BASE_URL}/flatcar_production_pxe_image.cpio.gz" -o "${DEST}/flatcar_production_pxe_image.cpio.gz"
+echo "flatcar_production_pxe_image.cpio.gz.sig"
+curl -# "${BASE_URL}/flatcar_production_pxe_image.cpio.gz.sig" -o "${DEST}/flatcar_production_pxe_image.cpio.gz.sig"
+
+# Install image
+echo "flatcar_production_image.bin.bz2"
+curl -# "${BASE_URL}/flatcar_production_image.bin.bz2" -o "${DEST}/flatcar_production_image.bin.bz2"
+echo "flatcar_production_image.bin.bz2.sig"
+curl -# "${BASE_URL}/flatcar_production_image.bin.bz2.sig" -o "${DEST}/flatcar_production_image.bin.bz2.sig"
+
+# Install oem image
+if [[ -n "${IMAGE_NAME-}" ]]; then
+  echo "${IMAGE_NAME}"
+  curl -# "${BASE_URL}/${IMAGE_NAME}" -o "${DEST}/${IMAGE_NAME}"
+  echo "${IMAGE_NAME}.sig"
+  curl -# "${BASE_URL}/${IMAGE_NAME}.sig" -o "${DEST}/${IMAGE_NAME}.sig"
+fi
+
+# verify signatures
+$GPG --verify "${DEST}/flatcar_production_pxe.vmlinuz.sig"
+$GPG --verify "${DEST}/flatcar_production_pxe_image.cpio.gz.sig"
+$GPG --verify "${DEST}/flatcar_production_image.bin.bz2.sig"
+
+# verify oem signature
+if [[ -n "${IMAGE_NAME-}" ]]; then
+  $GPG --verify "${DEST}/${IMAGE_NAME}.sig"
+fi