replace `readlink`

Fixes #182.

tools/flash.sh

diff --git a/tools/flash.sh b/tools/flash.sh
index f24127e..ea75dee 100755
--- a/tools/flash.sh
+++ b/tools/flash.sh
@@ -3,7 +3,7 @@
 set -eux
 
 artifact="$(basename $1)"
-rust_target_folder="$(readlink -f $(dirname $1)/../..)"
+rust_target_folder="$(cd $(dirname $1)/../.. && pwd -P)"
 if [ -z $APP_HEAP_SIZE ]; then
 	echo "Set APP_HEAP_SIZE to a value"
 	exit 1