pmb.types.CrossCompileType: split up None

Replace None with the three things it is actually used for to make the
code easier to follow:
* "autodetect"
* "unnecessary"
* "qemu-only"

I've used the term "unnecessary" instead of "native", because "native"
was previously used for "cross-native", then "cross-native2" and it
still sounds similar to these.

Part-of: !2568