utilize RTC on M5Core2

[Girgitt]

Currently sntp protocol is used to set the time however M5Core2 has a built-in RTC which could be used to speed-up booting.

Expected drift of about 1 min / month is good enough (www.switchdoc.com%2F2014%2F12%2Fbenchmarks-realtime-clocks-ds3231-pcf8563-mcp79400-ds1307%2F M5Core2's BM8563 is compatible with PCF8563 and maybe more than compatible..

[Girgitt]

On the other hand there are no issues with stability etc. in the last 2 months of 24/7 operation so this change does not necessary add anything and would possibly break something that works now: handling the DST change.