[system_updater] Adjust install-disk-image path

Recent changes to Go's exec.Command call me we must specify
the full path.

TEST=manual, validate that install-disk-image is started again by
OTA

PKG-155 #done

Change-Id: I46cec47dd7332047b37c26a18098a209bb503463
diff --git a/go/src/system_update_package/processor.go b/go/src/system_update_package/processor.go
index bd5f228..9f7db6d 100644
--- a/go/src/system_update_package/processor.go
+++ b/go/src/system_update_package/processor.go
@@ -157,14 +157,16 @@
 	return nil
 }
 
+var diskImagerPath = filepath.Join("/boot", "bin", "install-disk-image")
+
 func WriteImgs(imgs []string, imgsPath string) error {
 	for _, img := range imgs {
 		var c *exec.Cmd
 		switch img {
 		case "efi":
-			c = exec.Command("install-disk-image", "install-efi")
+			c = exec.Command(diskImagerPath, "install-efi")
 		case "kernc":
-			c = exec.Command("install-disk-image", "install-kernc")
+			c = exec.Command(diskImagerPath, "install-kernc")
 		default:
 			return fmt.Errorf("unrecognized image %q", img)
 		}