Change in coreboot[master]: util/abuild: Enable abuild to compile a single variant

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Change subject: util/abuild: Enable abuild to compile a single variant
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Patch Set 1: Verified+1

Build Successful

https://qa.coreboot.org/job/coreboot-checkpatch/29314/ : SUCCESS

https://qa.coreboot.org/job/coreboot-gerrit/75255/ : SUCCESS