Change in coreboot[master]: timestamp: Add function to get time since boot

[build bot (Jenkins) (Code Review)]

build bot (Jenkins) has posted comments on this change. ( https://review.coreboot.org/21516 )

Change subject: timestamp: Add function to get time since boot
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Patch Set 2: Verified+1

Build Successful

https://qa.coreboot.org/job/coreboot-checkpatch/15603/ : SUCCESS

https://qa.coreboot.org/job/coreboot-gerrit/60385/ : SUCCESS