- pm_off() should prevent other threads from getting executed after it is
called, as it should turn off the MCU completely according to its doc
- Previously, the fallback implementation would still allow other threads
to continue working
- Simply disabling IRQs and never enabling them again should make sure
the MCU behaves like it would be completely off
- pm_off() should reduce the power consumption as much as possible
- Previously, when IRQs came after the call to pm_set_lowest() in the
fallback implementation of pm_off(), `while(1) {}` got executed at full
power consumption
- Just calling `pm_set(0);` in a loop while make sure that lowest power mode
is restored if the MCU wakes up again.
- The check if the lowest power mode is available is skipped, as no code
gets executed afterwards anyway
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