libtime: measure overhead of libtime_nanosleep function call

This measurement allows us to account for that time when handling the sleep
request.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
libtime: remove implicit initialization in libtime_cpu_to_wall

Unnecessary and a bad idea.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>