Minor fix: Create root dir when it doesn't exist. (#5075)

realpath works differently on MacOS

Change-Id: I17e114cd289692aa6de8a5b4e6f29fc1734aca08

Author: freddan80

examples/arm/setup.sh

@@ -91,6 +91,7 @@ fi
 ### Optional user args
 ########
 root_dir=${1:-"${script_dir}/ethos-u-scratch"}
+mkdir -p ${root_dir}
 root_dir=$(realpath ${root_dir})
 
 ########
@@ -246,7 +247,6 @@ fi
 cd "${script_dir}"
 
 # Setup the root dir
-mkdir -p "${root_dir}"
 cd "${root_dir}"
 echo "[main] Using root dir ${root_dir}"