Meta: Assume and find GNU du

We want to use use the 'du' option '--apparent-size' which is a
GNU coreutils extension. GNU coreutils is a build dependency so
we know it is available. With this commit we first try to pick up
du as 'gdu', and if that fails, try 'du' instead.

Meta/build-image-qemu.sh

@@ -30,13 +30,17 @@ PATH="$SCRIPT_DIR/../Toolchain/Local/qemu/bin:$PATH"
 # directory was changed to Toolchain/Local/qemu.
 PATH="$SCRIPT_DIR/../Toolchain/Local/i686/bin:$PATH"
 
-disk_usage() {
- # shellcheck disable=SC2003
-if [ "$(uname -s)" = "Darwin" ]; then
- expr "$(du -sk "$1" | cut -f1)"
+# We depend on GNU coreutils du for the --apparent-size extension.
+# GNU coreutils is a build dependency.
+if type gdu > /dev/null 2>&1; then
+ GNUDU="gdu"
 else
- expr "$(du -sk --apparent-size "$1" | cut -f1)"
+ GNUDU="du"
 fi
+
+disk_usage() {
+ # shellcheck disable=SC2003
+ expr "$(${GNUDU} -sk --apparent-size "$1" | cut -f1)"
 }
 
 inode_usage() {