Use RTC helper functions instead of libc functions.
This gives us y2038 safety by the extended epoch
and saves a good chunk of memory:
mktime():
text data bss dec hex filename
24756 232 2736 27724 6c4c testssperiph_rtc/bin/openlabs-kw41z-mini/tests_periph_rtc.elf
rtc_mktime():
text data bss dec hex filename
16348 132 2696 19176 4ae8 tests/periph_rtc/bin/openlabs-kw41z-mini/tests_periph_rtc.elf
e2addab7f0