The cereal symbolizes the tasteless nature of Walt's actions.
Specifically, the raisins foreshadow Walt's descent into evil. As we know, water symbolizes the concept of life. Raisins are devoid of any water, similar to how Walt is devoid of his humanity. Walt is less so a human than a monster, and we see this resurface when
Jesse mentions to Hank and Gomez that "Mr. White is the devil" in season 5.
Walt, in many ways, is a raisin.
Additionally, the "crunch" represents Walt Jr.'s maturity and growth into Flynnhood. His obsession with cereal captures his innocence, but this particular cereal demonstrates his transition to a young adult.
Cereal in general can also be considered incomplete without milk, similar to how Hitler and his mustache were inseparable. This foreshadows
in season 5.
I hope this helps, Raz.