cc2540mini: Initialize process module

This patch initializes the process module and implements a simple
main loop, which process events and polls.
cc2540mini: Add blink example

This patch adds a simple blink example to illustrate how to program
an application to cc2540mini platform.
cc2540mini: Support for platform buttons

This patch implements support for cc2540mini B3 and B4 buttons and adds
an application which shows how to use them.