Patch the readlink command to work with busybox

--- old/git-flow
+++ new/git-flow
@@ -46,7 +46,7 @@
 # git-flow file is a symbolic link
 case $(uname -s) in
 Linux)
-	export GITFLOW_DIR=$(dirname "$(readlink -e "$0")")
+	export GITFLOW_DIR=$(dirname "$(readlink -f "$0")")
 	;;
 FreeBSD|OpenBSD|NetBSD)
 	export FLAGS_GETOPT_CMD='/usr/local/bin/getopt'