Examples: Add support for overriding lorawan keys during build via LORA_APPKEY, LORA_DEVEUI.. values

[plaes]

Implement lorawan key customization via environment variables (for nrf52840):
LORA_APPEUI= LORA_APPKEY= LORA_DEVEUI=1122334455667788 cargo build --release

[lthiery]

What about providing this as a util in the lorawan-device module? You could call the util from build.rs.

[lthiery]

Come to think of it, providing this as a util in the lorawan module will be difficult because this (and build.rs) can depend on std but lorawan can't.

We could create lorawan-util crates that has std... but I think this is also fine the way it is.

[lthiery]

Actually, I was wrong above. build-dependencies could have std enabled for lorawan-device which could provide this utility without having std enabled in the project itself. Then that could enable the from_str from lorawan-encoding (see review)?

[lthiery]

Some non-blocking suggestions.

[lthiery]

hex::encode?

[plaes]

Yeah, I initially didn't want to include extra dependency for just a single semi-trivial function (left-pad 🗡️ )..

[lthiery]

perhaps implement DevEui/AppEui/AppKey::from_str? Feature-gated behind std being enabled in lorawan-encoding.

[lthiery]

I also see that we hand-rolled the hex encoding here in lorawan-encoding. I've corrected there here.

[lthiery]

I went ahead and just made the PR for lorawan-encoding: #234

[lucasgranberg]

just for consideration the hex decoding can be done at compile time without custom build scripts by using the hex-literal crate. https://crates.io/crates/hex-literal

[plaes]

So, I basically ran into a headache and current version is good enough.
I can easily use the build.rs to parse the DevEui and friends, but in the end I would need to somehow "tell" rust to use this value in some way, which means I would have to either convert it back to array syntax or find something better.
This is how it works for now:

1. In generated file I have: const DEVEUI: Option<[u8; 8]> = None; (or Some([...]) which is easy to generate).
2. In lorawan example code I need to match the types so I can override: deveui: DevEui::from(DEVEUI.unwrap_or(DEFAULT_DEVEUI)),

(Also, from_str() requires pulling in use core::str::FromStr; and its a lot easier to type out zeroes as array than into hex string..)