Change get_timestamp() return type to int64_t to support ARMCC

[LDong-Arm]

time_t NTPClient::get_timestamp() returns negative values on errors, which is fine with GCC_ARM whose time_t is 64-bit integer. But the Arm Compiler's time_t is 32-bit unsigned integer, in ac6/include/time.h:

typedef unsigned int time_t;     /* date/time in unix secs past 1-Jan-70 */

When an error is reported, the value that gets returned with ARMCC is something like year 2100 and the caller cannot catch it.

To resolve this, change the return type of NTPClient::get_timestamp() to int64_t.
Note: This is a breaking change for applications compiled with the Arm Compiler. Callers should store the return value to a 64-bit integer, check the value, then cast it to time_t.

[evedon]

The implementation of get_timestamp assumes that negative numbers are returned for errors so we need to ensure that the returned type is signed.

[LDong-Arm]

@evedon Thanks for the review.
@0xc0170 As far as I know this library is used by some existing projects. This commit should be non-breaking for GCC_ARM (whose time_t is indirectly typedef'd to 64-bit signed integer), but in any case they should point to an existing SHA1 to avoid breakage?
Also, Travis failed with

$ git merge $TRAVIS_COMMIT
merge: e1681818e500c473e228edebdf60333c1fb51ce4 - not something we can merge
The command "git merge $TRAVIS_COMMIT" exited with 1.

which looks like a CI issue?

[0xc0170]

I've missed this earlier. Yes, the travis config is way to old.

[LDong-Arm]

I'm kind of inclined to change the API to

// Writes time to *time, returns true if suceeded
bool NTPClient::get_timestamp(time_t *time);

// Deprecated due to lack of error reporting, kept for compatibility
time_t NTPClient::get_timestamp();

[0xc0170]

@LDong-Arm if you rebase, I removed travis from this component. This shall be tested via application (as it can by its own), the way it was set up was using unsupported example. We should test but differently. I'll create an issue now with details.

[0xc0170]

@LDong-Arm if you rebase, I removed travis from this component. This shall be tested via application (as it can by its own), the way it was set up was using unsupported example. We should test but differently. I'll create an issue now with details.

created #16

[LDong-Arm]

@0xc0170 @evedon I've created #18 to change the API (as said previously), while keeping the old version of get_timestamp() for backward compatibility. I'm closing this one.