Change in coreboot[master]: payloads: Add LinuxBoot payload in u-root mode

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Change subject: payloads: Add LinuxBoot payload in u-root mode
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Patch Set 23: Verified-1

Build Unstable

https://qa.coreboot.org/job/coreboot-gerrit/69244/ : UNSTABLE

https://qa.coreboot.org/job/coreboot-checkpatch/23592/ : SUCCESS