xtask: Allow tasks & the kernel to specify default features.

[flihp]

tasks & kernels that want to disable default features can do so using the no-default-features boolean in the task / kernel config sections of the relevant app.toml files.

this resolves #1829

[flihp]

I'm sure there was a reason for disabling this so this may be ill advised but I want default features for the rng task in #1820. I've built a few images for the RoT and basic tire kicking was encouraging. The output from the build (task & kernel elfs) are identical except for the idle task. I haven't dug deep into why but if anyone is interested in this feature or thinks this is a bad idea I'd appreciate the feedback good or bad.

[flihp]

As a way to convince myself that enabling default-features doesn't have any unintended side effects the first commit in this PR removes all use of default-features from existing tasks. These were no-ops since we were passing --no-default-features to rustc anyway. The second commit here enables the use of default-features in tasks and because we removed all use of these in the previous commit the build output is identical both before and after dc75180 is applied. I have tested and confirmed that this is the case for every app.toml in the source tree.

[cbiffle]

Historical context:

So, in the early days, we used outside-of-xtask builds in two situations:

1. Doing single-task iteration (in the days before xtask build) using cargo check
2. In editors using RLS, back when RLS was a thing

We used default features to fake a subset of the build system features to achieve this, which is part of what you're finding in the old default features lists.

Nowadays we've got the better rust-analyzer - build-system integration, and it's probably safe to stop doing this hack. So I'm generally positive about this change in concept, though I haven't finished reading the implementation yet.