Keep in mind that's mainly because the ARM world is not standardized the way the PC world is. In practice the differences between most of these targets will be

Keep in mind that's mainly because the ARM world is not standardized the way the PC world is. In practice the differences between most of these targets will be the very early initialization code and things like the clock hierarchy but beyond that most of the code will be shared across many variants. Imagine having to use a different kernel every time you upgrade your desktop, that's basically how the ARM world works so far.