Why I did it
SONiC service determine-reboot-cause might run before driver creating reset cause files. In that case, the reset cause will be "Unknown". This PR introduces a wait mechanism to wait for reset cause sysfs files ready.
How I did it
/run/hw-management/config/reset_attr_ready is the file to indicate all reset cause files are ready. In chassis.get_reboot_cause function, it waits /run/hw-management/config/reset_attr_ready for up to 45 seconds.
How to verify it
Manual test on master/202211/202205
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