main/postmarketos-mkinitfs: change wait for subpartitions

Previously it was assumed that with the counter and the sleep 0.1 the
process would take about 10 seconds to complete. On newer devices with
tens or even hundreds of partitions going through all partitions already
takes a significant amount of time, so change the logic to measure the
time elapsed instead.

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It's worth noting that I did consider using date for counting the seconds but given we're in the initramfs where probably the time is not set yet (or will be changed, whatever), I think the uptime solution is more robust.