Introduce cmake-based rewrite of KBuild

Introducing CMake is an important step in a larger effort to make
Zephyr easy to use for application developers working on different
platforms with different development environment needs.

Simplified, this change retains Kconfig as-is, and replaces all
Makefiles with CMakeLists.txt. The DSL-like Make language that KBuild
offers is replaced by a set of CMake extentions. These extentions have
either provided simple one-to-one translations of KBuild features or
introduced new concepts that replace KBuild concepts.

This is a breaking change for existing test infrastructure and build
scripts that are maintained out-of-tree. But for FW itself, no porting
should be necessary.

For users that just want to continue their work with minimal
disruption the following should suffice:

Install CMake 3.8.2+

Port any out-of-tree Makefiles to CMake.

Learn the absolute minimum about the new command line interface:

$ cd samples/hello_world
$ mkdir build && cd build
$ cmake -DBOARD=nrf52_pca10040 ..

$ cd build
$ make

PR: zephyrproject-rtos#4692
docs: http://docs.zephyrproject.org/getting_started/getting_started.html

Signed-off-by: Sebastian Boe <sebastian.boe@nordicsemi.no>

CMakeLists.txt

@@ -0,0 +1 @@
+add_subdirectory_ifdef(CONFIG_HAS_SILABS_GECKO gecko)

gecko/CMakeLists.txt

@@ -0,0 +1,29 @@
+# Makefile - Gecko SDK
+#
+# Copyright (c) 2017, Christian Taedcke
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+# Translate the SoC name and part number into the gecko device and cpu name
+# respectively.
+string(TOUPPER ${CONFIG_SOC_SERIES} SILABS_GECKO_DEVICE)
+
+set(SILABS_GECKO_PART_NUMBER ${CONFIG_SOC_PART_NUMBER})
+
+zephyr_include_directories(
+  Device/SiliconLabs/${SILABS_GECKO_DEVICE}/Include
+  emlib/inc
+  )
+
+# The gecko SDK uses the cpu name to include the matching device header.
+# See Device/SiliconLabs/$(SILABS_GECKO_DEVICE)/Include/em_device.h for an example.
+zephyr_compile_definitions(
+  ${SILABS_GECKO_PART_NUMBER}
+  )
+
+zephyr_sources(                                emlib/src/em_system.c)
+zephyr_sources_ifdef(CONFIG_HAS_CMU            emlib/src/em_cmu.c)
+zephyr_sources_ifdef(CONFIG_GPIO_GECKO         emlib/src/em_gpio.c)
+zephyr_sources_ifdef(CONFIG_SERIAL_HAS_DRIVER  emlib/src/em_usart.c)
+zephyr_sources_ifdef(CONFIG_SOC_SERIES_EFM32WG Device/SiliconLabs/EFM32WG/Source/system_efm32wg.c)