Add update script

update.sh

+#!/usr/bin/env bash
+set -e
+
+source config.sh
+
+if [ $# = 0 ]
+then
+    recipes="$(ls -1 recipes)"
+else
+    recipes="$@"
+fi
+
+for recipe in $recipes
+do
+    if [ -d "recipes/$recipe/build" ]
+    then
+        ./cook.sh "$recipe" update
+    fi
+done