First of all, you need to know that breakfast does not have to be eaten immediately after waking up. You may as well have it right after your training. If you exercise really early and do not want to wake up at dawn only to have a bowl of cereal or squeeze in a sandwich, know that you do not have to do this! As an alternative, however, I suggest buying BCAA: branched amino acids supplement, preferably powder to dilute in water. One serving (approx. 5-10 g), possibly supplemented with a dose of caffeine, can be eaten just after you wake up, that is optimally about 20-40 minutes before exercise, and the other (also 5-10 g) during or after exercise. I myself was repeatedly doing morning workouts having only a portion of amino acids. They are comfortable, they do not burden the stomach and protect muscles against catabolism.
If you exercise a little later, or you cannot imagine training on an empty stomach, I suggest having a light breakfast. In case that between your meal and training there is just a small break (30-60 minutes), I suggest a light meal consisting of milk rice (cooked in the evening the day before) with dried or fresh fruits or a banana cocktail which can also be prepared with milk. Both rice and milk are an easily digestible source of nutrients and do not burden your digestive tract. If so far you have had porridge or muesli and felt discomfort during your training, changing your breakfast will allow remedy.
A good and extremely convenient solution are protein and carbohydrate-protein nutrients. It is true that this is a highly processed food, but while you train, it suits you ideally. I strongly recommend whey proteins which are easily digestible and can be readily managed by your stomach. The issue of choosing carbohydrates is highly individual because proteins contained in these supplements quite often resemble simple sugars in their "behaviour", so having too much may not be desirable for most people. Especially in the period focused on fat tissue reduction I suggest not having too much of pre-training carbohydrates. If you decided to buy whey protein and carbohydrates separately, you can sip the latter during the workout.
The common belief, according to which you need to eat well to have strength to train, is a myth which absolutely should not be given any credit! Many people, unfortunately, believe it, and consequently perform morning sessions with a full stomach after a hearty breakfast. I have proposed some simple solutions, which are an interesting alternative for all those who for some reasons took a liking to morning workouts.