Does anyone know if it's possible to specify the build target via cargo features?
Instead of `cargo build --features bsp_rpi4b --target aarch64-unknown-none`, I would like to specify just `cargo build --features bsp_rpi4b`, and have the mapping to `aarch-64-unknown-none` target specified in `Cargo.toml`.

I've tried:
```toml
# Cargo.toml
[features]
default = [] # Host ISA
bsp_rpi4b = [] # BCM2711 (aarch64)
bsp_sifive_unmatched = [] # FU740-MC (RISC-V64GC)

[target.'cfg(feature = "bsp_rpi4b")']
target = "aarch64-unknown-none"

[target.'cfg(feature = "bsp_sifive_unmatched")']
target = "riscv64-unknown-none-elf"
```
but the compiler is mad at me:
```
warning: Found `feature = ...` in `target.'cfg(...)'.dependencies`. This key is not supported for selecting dependencies and will not work as expected. Use the [features] section instead: https://doc.rust-lang.org/cargo/reference/features.html
warning: Found `feature = ...` in `target.'cfg(...)'.dependencies`. This key is not supported for selecting dependencies and will not work as expected. Use the [features] section instead: https://doc.rust-lang.org/cargo/reference/features.html
warning: unused manifest key: target.cfg(feature = "bsp_rpi4b").target
warning: unused manifest key: target.cfg(feature = "bsp_sifive_unmatched").target
```
Any thoughts?