all: make targets configurable with a JSON file

This is intentionally similar to the target specifications in Rust:
https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.TargetOptions.html

target.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// Target specification for a given target. Used for bare metal targets.
+//
+// The target specification is mostly inspired by Rust:
+// https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.TargetOptions.html
+// https://github.com/shepmaster/rust-arduino-blink-led-no-core-with-cargo/blob/master/blink/arduino.json
+type TargetSpec struct {
+	Triple    string   `json:"llvm-target"`
+	BuildTags []string `json:"build-tags"`
+}
+
+// Load a target specification
+func LoadTarget(target string) (*TargetSpec, error) {
+	spec := &TargetSpec{
+		Triple:    target,
+		BuildTags: []string{runtime.GOOS, runtime.GOARCH},
+	}
+
+	// See whether there is a target specification for this target (e.g.
+	// Arduino).
+	path := filepath.Join("targets", strings.ToLower(target)+".json")
+	if fp, err := os.Open(path); err == nil {
+		defer fp.Close()
+		err := json.NewDecoder(fp).Decode(spec)
+		if err != nil {
+			return nil, err
+		}
+	} else if !os.IsNotExist(err) {
+		// Expected a 'file not found' error, got something else.
+		return nil, err
+	} else {
+		// No target spec available. This is fine.
+	}
+
+	return spec, nil
+}