cc2540mini: Add support for platform buttons

This patch implements support for cc2540mini B3 and B4 buttons using
Contiki sensor feature.

Signed-off-by: Paulo Borges <paulo.borges@openbossa.org>
cc2540mini: Add platform buttons example

This patch adds an example which handles with both buttons present
in the platform.

Signed-off-by: Paulo Borges <paulo.borges@openbossa.org>