Allow environment variables to set chip arguments.

README.md

@@ -33,13 +33,17 @@ Add this line to your Cargo configuration (`.cargo/config`) file:
 
 ``` toml
 [target.'cfg(all(target_arch = "arm", target_os = "none"))']
-runner = "probe-run --chip $CHIP"
+runner = "probe-run --chip ${PROBE_RUN_CHIP}"
 ```
 
-Instead of `$CHIP` you'll need to write the name of your microcontroller.
+Instead of `${PROBE_RUN_CHIP}` you can write the name of your microcontroller.
 For example, one would use `nRF52840_xxAA` for the nRF52840 microcontroller.
 To list all supported chips run `probe-run --list-chips`.
 
+To support multiple devices, or permit overriding default behavior, you may prefer to set the
+`${PROBE_RUN_CHIP}` environment variable, and set `runner` (or
+`CARGO_TARGET_${TARGET_ARCH}_RUNNER`) to `probe-run`.
+
 2. Enable debug info
 
 Next check that debug info is enabled for all profiles.

src/main.rs

@@ -51,7 +51,7 @@ struct Opts {
     defmt: bool,
 
     /// The chip to program.
-    #[structopt(long, required_unless("list-chips"))]
+    #[structopt(long, required_unless("list-chips"), env = "PROBE_RUN_CHIP")]
     chip: Option<String>,
 
     /// Path to an ELF firmware file.