Meta: Improve build compatibility with non-GNU userspaces

We depend on GNU-specific du switch `--apparent-size`. Busybox has this
implemented, but as `-b` instead.
Another part of the build system uses `cp --preserve=timestamps`. This
can be replaced by `rsync -t`, and rsync is already used through the
file.

Thanks to those changes, Serenity can be built on a Busybox system,
without GNU coreutils.

Meta/build-image-extlinux.sh

@@ -35,14 +35,6 @@ if [ -z $syslinux_dir ]; then
     exit 1
 fi
 
-disk_usage() {
-if [ "$(uname -s)" = "Darwin" ]; then
-    du -sm "$1" | cut -f1
-else
-    du -sm --apparent-size "$1" | cut -f1
-fi
-}
-
 DISK_SIZE=$(($(disk_usage "$SERENITY_SOURCE_DIR/Base") + $(disk_usage Root) + 300))
 
 echo "setting up disk image..."