Just like `tests/periph_rtt`, delay the RTC init so that if init gets stuck
it doesn't get stuck in early boot but inside the test.
This makes debugging easier.
`tests/periph` calls the functions `rtc_get_alarm` and `rtc_set_alarm` from an ISR. Depending on the implementation of `periph_rtc`, however, these functions use `newlib` functions such as `localtime` and `mktime`, which in turn call the `newlib` lock function `__tz_lock`. This can lead to a system lock if the `newlib` with real lock functionality is used instead of the `newlib_nano`.
A naive implementation may set a RTC alarm in 30s by calling
struct tm now;
rtc_get_time(&now);
now.tm_sec += 30;
rtc_set_alarm(&now, _cb, NULL);
This works for RTC implementations that use a RTT internally and call
mktime() to convert the struct tm to a unix timestamp, as mktime() will
normalize the struct in the process.
Call rtc_tm_normalize() when the RTC uses separate registers for time / date
components to ensure it is normalized.
This also modifies tests/periph_rtc to exercise this case.
