platform(d1): introduce config for the D1

[hawkw]

Closes #217

This branch introduces mnemos-config support for the Allwinner D1 platform, building on @jamesmunns' work in #217. I've made a few changes from James' initial implementation. In particular, I've changed how the board config is determined. Now, rather than using feature flags to determine what board is being built, we once again use bin targets. However, each bin target now has a very simple main function that simply selects which config file to include. For example, the lichee-rv.rs bin target now just looks like this:

#![no_std]
#![no_main]

#[allow(non_snake_case)]
#[riscv_rt::entry]
fn main() -> ! {
    let config = mnemos_config::include_config!(mnemos_d1::PlatformConfig, "lichee-rv").unwrap();
    mnemos_d1::kernel_entry(config);
}

[hawkw]

bleh i guess CI is wedged