solo5: should not set booted_at to the new reboot time after a solo5_exec

[ricarkol]

When doing a solo5_exec we are resetting uptime and micros_since_boot. This is wrong as micros_since_boot should measure the time since the first boot (not the reboot due to the exec).
This change fixes that by not setting the booted_at_ variable at every boot; instead, it leaves it at 0. This is a good approximation as booted_at_ usually has less than 1 ms (~700000 nanoseconds).

@fwsGonzo ^^^

[jenkins-includeos]

Can one of the admins verify this patch with one of the following commands:

• "jenkins test please" for a one time test run
• "ok to test" to accept latest and future commits on this pull request for testing
• "add to whitelist" to add the author to the whitelist