this helps minimal configurations to be more useful without pulling
in com/llio crates. The trade-off is that KP's early in boot
(or during resume) are not visible, but, these are (thankfully??)
less rare these days.
This will allow for e.g. xous on riscv32-imac but not a precursor
target, for example: by using feature selectors we get to have
a bigger range of targets that we support, such as for implementations
that haven't been upstreamed or have official target-triples.
The downsides are:
- patch override libraries don't absorb feature flags. so, for
example, the AES crate that is used to patch other crates won't
be aware of the target features without modifying the referencing
crates.
- features are normally meant to be additive, but target features
are mutually exclusive. The mutex is enforced by a check in
utralib that throws a panic on build if two targets are accidentally
selected at the same time.
for now, publish a xous-riscv package, until we can absorb
this into the kernel directly.
Want to recode all the assembly files into inline-assembly now
that it is supported in Rust, but, let's not put that yak on
the list of yaks to shave before we have a separable kernel.
- strip out gdb-stub. Per xobs, the feature never worked, and it
depends upon a branch of a git repo which is not allowed by crates.io
Since the feature hasn't been developed, and crates.io will not
allow even an optional feature that points to a git repo, the feature
was stripped out.
- rename "kernel" to "xous-kernel", so that we can live in a crates.io
package namespace that is specific to Xous.
still have to figure out the riscv package issue.
