Add commands to check if actual time is in DST or not

[MaxineMuster]

New command clock_calcDST nthWeekEnd monthEnd dayEnd hourEnd nthWeekStart monthStart dayStart hourStart
to set the DST start and end rule.

Examples:
Australia / Sydney CLOCK_calcDST 1 4 1 3 1 10 1 2
Amsterdam (EU) CLOCK_calcDST 0 10 1 3 0 3 1 2

$isDST will return 1 if DST is active, 0 else

It's enabled with

#define ENABLE_NTP_DST 1

[giedriuslt]

careful with this, I see it's already to big for bl602, probably because of strftime use

[MaxineMuster]

probably because of strftime use

That could be easily solved, it's only used for informational purposes. And in any case we don't need to enable this on BL602.

It's really time my BL602 arrives, so I can do my own tests...

Is this a result from testing or is there a known size limit?

[openshwprojects]

I think I can merge it, but can you disable it by default, since it seems to raise flash uasge a bit?