rp235x firmwares need an Image Definition if they want to be called
from the rp235x bootrom.
Currently this Image Definition is manually added to each project/example,
but for most users it will always be the default (Secure Exe).
This commit adds crate features to allow users to configure this, with the
default of including a Secure Exe Image Definition in.
Just like the boot2-* features, this includes an opt-out (imagedef-none)
to allow the user to not make use of this included Image Definition.
4cc5ab9474