cc2540mini: Initial support for CC2540 mini development kit

This patch add the initial files for supporting CC2540 mini
development kit from TI.
cc2540mini: Add LED support for CC2540
cc2540mini: Add putchar()

Each platform should define its own putchar() according to platform
characteristics.

Since cc2540mini porting is in early stage, we define an empty
putchar() so we are able to compile hello-world example.
Temporary HACK

Since we don't support printf() yet, we need this hack to compile
hello-world.c example.
cc2540mini: Add cc2540mini build to .gitignore
cc2540mini: Initialize clock module

Since cc2540 and cc253x shares same SFR for clock configuration, we can safely
call cc253x's implementation. We also need to call soc_init() because it
activates the global interruption flag (EA).
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.
cc2540mini: Initialize etimer process
cc2540mini: Modify blink example to use etimer

This patch modifies the blink example to use etimer instead of simple
timer. This allows the blink process to yield, cooperating with other
processes.
cc2540mini: Initialize ctimer module
cc2540mini: Initialize rtimer module