esp32: support flashing directly from tinygo

Right now this requires setting the -port parameter, but other than that
it totally works (if esptool.py is installed). It works by converting
the ELF file to the custom ESP32 image format and flashing that using
esptool.py.

builder/build.go

@@ -279,6 +279,13 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(stri
 			if err != nil {
 				return err
 			}
+		case "esp32":
+			// Special format for the ESP32 chip (parsed by the ROM bootloader).
+			tmppath = filepath.Join(dir, "main"+outext)
+			err := makeESP32FirmareImage(executable, tmppath)
+			if err != nil {
+				return err
+			}
 		default:
 			return fmt.Errorf("unknown output binary format: %s", outputBinaryFormat)
 		}