If they even ask in that way, just say what you wrote “I want to immigrate and find a workplace” or simply “I am coming to the US to find work and move back to the country”. As a citizen it's your right, you have no reason to try to pigeonhole what you are doing in a category like “business”.

Generally speaking, it's important to be genuine and always remain truthful. In this case, it's very easy since, as a citizen, it is your right to move back to the country. Unlike aliens trying to meet the conditions of their visa, you have no reason to try to pigeonhole what you are doing in a category like “business”. So if you are asked about the purpose of your trip, just say what you wrote “I want to immigrate and find a workplace” or simply “I am coming to the US to find work and move back to the country”.

In this scenario, CBP officers are more interested in making sure that you are who you say you are, enforcing customs rules and possibly detecting criminals than in the purpose of your stay as this is irrelevant to the entry decision for US citizens. They are just as likely to ask where you have been as where you are going. Do make sure you have a valid US passport to establish your citizenship and you will be fine.