In multi-core environment where NuttX runs on each core, if one core
sets the RTC time, the RTC value gotten on other cores is incorrect.
This is caused by clock_gettime(CLOCK_MONOTONIC) function used to get
elapsed time, which uses a core-specific global varaiable g_basetime
as the base time.
To fix this, update the g_basetime from the backup SRAM that can be
shared between cores in setting/getting the RTC time.
Signed-off-by: SPRESENSE <41312067+SPRESENSE@users.noreply.github.com>
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