I have some interest in each of these areas, and given enough time and effort, I believe I could work my way up to an acceptable level of ability. However, I don't really have a good pulse on how these industries are going and what it's actually like to work in one of these fields. Here are my preliminary thoughts about each:
Embedded linux engineer: +field doesn't change quickly, is similar to my current embedded work, which I do enjoy -might get boring; kernel debugging sounds somewhat stressful
Pentester/reverse engineering: +very exciting work, lots of interesting tools and sometimes assembly (I love looking at ARM disassembly and poking registers!) -minimum bar might be exceedingly high, making it hard to break into the field -may not be very stable as a career choice
Robotics engineer: +many interesting applications of machine learning and state-of-the-art techniques -requires a hardcore math background to understand things at a deep level (I've only studied up to intro-level linear algebra) -field changes rapidly every single year (hello CNNs)
Do you work in one of these fields? What is work like for you day-to-day? Where do you see your industry going, and do you believe you've made a good long-term career choice?