builder: implement Nordic DFU file writer in Go

This avoids a dependency on nrfutil. I have verified that it creates
equivalent zip files to a wasp-os DFU zip file I downloaded here:
https://github.com/wasp-os/wasp-os/releases/
I have also tested that it produces valid DFU files that can be uploaded
using the dfu.py program here to my PineTime:
https://github.com/wasp-os/ota-dfu-python/tree/3d6fd30d33c2b20bc86ff6b9269fddf4a1d4c7c6

There are some minor differences in the generated file that should not
matter in practice (JSON whitespace, firmware file name, zip
compression).

builder/build.go

@@ -909,13 +909,8 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		}
 	case "nrf-dfu":
 		// special format for nrfutil for Nordic chips
-		tmphexpath := filepath.Join(tmpdir, "main.hex")
-		err := objcopy(result.Executable, tmphexpath, "hex")
-		if err != nil {
-			return result, err
-		}
 		result.Binary = filepath.Join(tmpdir, "main"+outext)
-		err = makeDFUFirmwareImage(config.Options, tmphexpath, result.Binary)
+		err = makeDFUFirmwareImage(config.Options, result.Executable, result.Binary)
 		if err != nil {
 			return result, err
 		}