buildroot: Compress images with zstd

My benchmarks show that zstd has the second best decompression
ratio/speed, whereas the best decompression ratio algorithm is
not the same as the best decompression speed algorithm. Compression
speed is not awful either.

Check that the tool is installed and bail out if it is not.

Link: ClangBuiltLinux/continuous-integration#184 (comment)
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

buildroot/rebuild.sh

@@ -23,6 +23,9 @@ set -u
 # Move into the folder that contains this script
 cd "$(dirname "$(readlink -f "${BASH_SOURCE[0]}")")" || exit 1
 
+# Make sure the user has zstd installed
+command -v zstd &>/dev/null || die "zstd could not be found on your system, please install it!"
+
 # Generate list of configs to build
 CONFIGS=()
 [[ ${#} -eq 0 ]] && die "Please specify the configs that you want to build as parameters to this script!"
@@ -75,6 +78,6 @@ for CONFIG in "${CONFIGS[@]}"; do
     IMAGES=( "output/images/rootfs.cpio" )
     for IMAGE in "${IMAGES[@]}"; do
         [[ -f ${IMAGE} ]] || die "${IMAGE} could not be found! Did the build error?"
-        cp -v "${IMAGE}" "${IMAGES_FOLDER}"
+        zstd -19 "${IMAGE}" -o "${IMAGES_FOLDER}/${IMAGE##*/}.zst" || die "Compressing ${IMAGE##*/} failed!"
     done
 done