Fix delay implementation

[AntonioLangiu]

I noticed that while performing some operations (such as signature verification with ATECC508a) the final read used to get the final value fails. I think the problem is a delay problem due to the implementation of the timer. In fact, adding a delay during the initial implementation fixes the problem. However I need to investigate more to find a more reliable solution, maybe implementing the delay functions using directly the timer hw of the CC2650.

To finish this issue I should measure if:

• The delay implemented using the rtimer is working;
• Create another delay implementation using directly the driverlib/timer.