certify_bootimg: support spaces in extra_args am: b771593b6c am: b0aa54e6b8 am: 328397e831

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2033665

Change-Id: I0fa731fab8c5e69e2c660c9d3dfdb05959d85564