cc2540mini: Initial support for CC2540 mini development kit

This patch adds the initial files for supporting CC2540 mini
development kit from TI. Since clock module and etimer library
is already supported by the SoC, we initialize etimer and ctimer
processes, as well as autostart process.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
cc2540: rtimer module initial support

Since the SoC dependent file rtimer-arch.h is required to build
Contiki, we add the inital support for rtimer module. By now,
we use the system clock module to implement the rtimer module.
Nevertheless, the rtimer module will be properly support in future.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
cc2540: Add mtarch.h header

This patch adds the mtarch.h header which defines a dummy struct
mtarch_thread in order we are able to build Contiki for cc2540mini
platform.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
cc2540mini: Add LED support

This patch adds the LED support for cc2540mini platform. The
cc2540mini platform has only two LEDs (green and red).

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
cc2540: Change linker options

This patch changes the linker --xram-loc option so the start location
of the external RAM is 0x0001. This way we prevent the NULL address
to be used as a valid memory address.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
examples: Add blink example

This patch adds a simple blink example which inverts the green led
each second.

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