certify_bootimg: support spaces in extra_args am: b771593b6c am: b0aa54e6b8 am: 328397e831 am: 038ee4f1d3

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

Change-Id: I5f74e37ccf047cd60a4627f25f548001be0861d9