Add target specification to rust-toolchain file

[refugeesus]

Describe the problem you are trying to solve
I would like to add a target specification as a sub-category of the toolchain specified in the rust-toolchain file to automatically fetch the target for a toolchain if not already added.

Describe the solution you'd like
I think it would be helpful to automatically fetch a specified target if it is not currently installed when building projects containing a rust-toolchain file. Allowing for a specific target sub-category under the toolchain definition would remove build errors due to fetching a new toolchain set in the rust-toolchain file which then would not have the associated target. This isn't a huge issue, just an ease of use improvement particularly for users who are not familiar with compiler components.

Notes
Anyone starting out building projects using rustup without prior knowledge of compiling projects for embedded systems is going to trip over this concept for a while initially. The argument can be made that this is "part of the learning process" but setting up the rust-toolchain file with a target implies understanding that the components were needed. This is mainly for collaborative work where people are working across multiple projects with several toolchains/targets.

[kinnison]

I'm not entirely sure that rust-toolchain is really the right place for this, though I also can't think quite of a better option right now.

How would you envisage the file being altered, and what would the effect be if the target in question wasn't available for the given toolchain? What if they run cargo build --target=anothertarget instead of just cargo build etc?

[jschwe]

I think #2438 closes this.