add example pico_w/wifi/ntp_system_time

[mjcross]

A proposed new NTP example that differs from the existing pico_w/wifi/ntp_client in the following ways:

1. uses the SNTP app already provided by lwIP (in threadsafe background mode)
2. uses pico_aon_timer to create a time-of-day clock that can be read asynchronously from user code
3. illustrates the use of a POSIX timezone (TZ) to convert UTC to local time with daylight saving
4. includes a README.md to explain what the example does and how to use it

The program connects to Wi-Fi, initialises the lwIP SNTP app, syncs the pico_aon_timer to NTP and then enters a loop to display the local time in London (or UTC, or whichever timezone the user defines); resynchronising to ntp.pool.org in the background every hour.

I suggest it's quite an important use case for the Pico-W, and illustrates a couple of features users may find helpful (such as how to create and use a POSIX timezone).

[lurch]

Hmmm, if #716 and #714 got together and had a baby... :thinking: :joy:

[mjcross]

Hmmm, if #716 and #714 got together and had a baby... 🤔 😂

https://digitalcollections.smu.edu/digital/collection/tir/id/205/

[lurch]

Thanks for the extra comments :+1:
Not having done any time-based processing in C myself, I was misreading asctime() as ascii_time() whereas I guess it's actually meant to be read as as_ctime() :joy:

[mjcross]

I was misreading asctime() as ascii_time() whereas I guess it's actually meant to be read as as_ctime() 😂

Oooo never considered that... My "lengthy and extensive research" (ha ha) suggests it might be the former: https://retrocomputing.stackexchange.com/questions/25601/what-is-the-meaning-of-asctime In fact for anything more complex than a timestamp it's probably better to use strftime() but that's a bit OTT for an embedded app :-)

[mjcross]

Found a way to avoid hardcoding the timezone names in the printf() :-)

[mjcross]

Many thanks for the excellent suggestions - should be all wrapped up in the latest commit (please let me know if I missed anything)

[mjcross]

Looking really good now - many thanks again for your time to review

[mjcross]

Spent far too long today chasing down why some simple test code couldn't find the function definition for setenv(). Eventually realised it was because in the example I'd forgotten to #include <stdlib.h>, and got away with it because lwIP pulls it in. Hopefully by importing it explicitly it'll save someone else from the same fate...