radio: Add intial radio files

This patch adds the initial platform specific files, related to radio
interface function. Each platform should implement its abstractions
based on the radio header file.
build: Add initial autotools support
build: Add build files for NRF platform

This patch adds the initial Makefiles and autotools files to build a
hello world example. Initial configure options was added to allow
selecting different platforms.

Usage example:
./bootstrap-configure.nrf --with-sdkdir=/osd/work/iot/nrf51-sdk.git
examples: Remove platform subdir for Hello World
build: Add Makefile per example

This patch changes the build system creating one Makefile for each project.
build: Set heap size to zero

Work in progress
build: Fix warning treaded as error

In file included from simple_uart.c:17:0:
/osd/work/iot/nrf51-sdk.git/Nordic/nrf51822/Include/nrf_delay.h:48:1:
error: 'inline' is not at beginning of declaration
[-Werror=old-style-declaration]
nrf51822: Add local copy of platform files
radio: Change radio_init() return type
nrf51822: Add radio.c to Makefile.platform
WIP: radio: Define and implement Radio APIs