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.