libtock: sys: Implement _gettimeofday

[alistair23]

Implement the _gettimeofday() function, which fixes build issues with C code that uses the gettimeofday syscall.

[bradjc]

What would a caller do with the results from this function? I'm a little wary of implementing this in the core library and having it not really do the correct thing. Plus we do have a RTC interface, so in theory this could use the RTC and return a valid timestamp.

[alistair23]

A caller can use this to see time ticking basically.

The idea is that there are existing libraries which use gettimeofday(). If we don't implement it they will fail to link as we don't have an implementation. This provides one

[bradjc]

Seems like there are two competing ills: not having _gettimeofday() causing compilation failures, and having gettimeofday() not actually return the time of day (when a user might expect it to). I think I would be frustrated to find out that gettimeofday() compiles but doesn't actually do what the docs say it does.

The simple solution to my issue is just to have each library implement _gettimeofday() according to whatever requirements that library needs (perhaps just returning zeros is ok, maybe ticks is ok).

I would be supportive of a middle ground, where we keep the implementation in this PR but call it something else (maybe gettimeasticks() or something), and then have individual libraries include the code:

int _gettimeofday(struct timeval *tv, void *tzvp) {
  return gettimeasticks(tv, tzvp);
}

if they want.

[alistair23]

This is ready to merge