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>

kernel/CMakeLists.txt

@@ -0,0 +1,45 @@
+# kernel is a normal CMake library and not a zephyr_library because it
+# should not be --whole-archive'd
+add_library(kernel
+  alert.c
+  device.c
+  errno.c
+  idle.c
+  init.c
+  mailbox.c
+  mem_slab.c
+  mempool.c
+  msg_q.c
+  mutex.c
+  pipes.c
+  queue.c
+  sched.c
+  sem.c
+  stack.c
+  sys_clock.c
+  system_work_q.c
+  thread.c
+  thread_abort.c
+  version.c
+  work_q.c
+)
+
+target_sources_ifdef(CONFIG_INT_LATENCY_BENCHMARK kernel PRIVATE int_latency_bench.c)
+target_sources_ifdef(CONFIG_STACK_CANARIES        kernel PRIVATE compiler_stack_protect.c)
+target_sources_ifdef(CONFIG_SYS_CLOCK_EXISTS      kernel PRIVATE timer.c)
+target_sources_ifdef(CONFIG_LEGACY_KERNEL         kernel PRIVATE legacy_timer.c)
+target_sources_ifdef(CONFIG_ATOMIC_OPERATIONS_C   kernel PRIVATE atomic_c.c)
+target_sources_ifdef(CONFIG_PTHREAD_IPC           kernel PRIVATE pthread.c)
+target_sources_if_kconfig(                        kernel PRIVATE poll.c)
+
+target_sources_ifdef(
+  CONFIG_USERSPACE
+  kernel PRIVATE
+  userspace.c
+  userspace_handler.c
+  mem_domain.c
+  )
+
+add_dependencies(kernel offsets_h)
+
+target_link_libraries(kernel zephyr_interface)