Fixes for compiler warnings in pedantic mode.

Includes static function fix and all the previous improvements and fixes
by @dpgeorge.

This prevents duplication of objects in the sleep list. Also helps
with reducing the code size by ~100 bytes.

Most boards (except the pyboard) don't have a 32kHz crystal so they
should use the LSI for the RTC.

We already have branch for lookup->is_type == true, so here it's guaranteed
to be false.

According to the new SDK (1.1.0) this is not needed, and it's best
not to do it, because this module is a shared resource.

This allows to wake from low-power modes at a regular interval.

This method is preliminary, pending testing and API overhaul.