kernel: Scheduler rewrite

This replaces the existing scheduler (but not priority handling)
implementation with a somewhat simpler one.  Behavior as to thread
selection does not change.  New features:

+ Unifies SMP and uniprocessing selection code (with the sole
  exception of the "cache" trick not being possible in SMP).

+ The old static multi-queue implementation is gone and has been
  replaced with a build-time choice of either a "dumb" list
  implementation (faster and significantly smaller for apps with only
  a few threads) or a balanced tree queue which scales well to
  arbitrary numbers of threads and priority levels.  This is
  controlled via the CONFIG_SCHED_DUMB kconfig variable.

+ The balanced tree implementation is usable symmetrically for the
  wait_q abstraction, fixing a scalability glitch Zephyr had when many
  threads were waiting on a single object.  This can be selected via
  CONFIG_WAITQ_FAST.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

kernel/include/kernel_structs.h

@@ -12,6 +12,7 @@
 #if !defined(_ASMLANGUAGE)
 #include <atomic.h>
 #include <misc/dlist.h>
+#include <misc/rb.h>
 #include <string.h>
 #endif
 
@@ -27,7 +28,6 @@
  * defined.
  */
 
-
 /* states: common uses low bits, arch-specific use high bits */
 
 /* Not a real thread */
@@ -48,6 +48,9 @@
 /* Thread is actively looking at events to see if they are ready */
 #define _THREAD_POLLING (1 << 5)
 
+/* Thread is present in the ready queue */
+#define _THREAD_QUEUED (1 << 6)
+
 /* end - states */
 
 #ifdef CONFIG_STACK_SENTINEL
@@ -69,13 +72,13 @@ struct _ready_q {
 #ifndef CONFIG_SMP
 	/* always contains next thread to run: cannot be NULL */
 	struct k_thread *cache;
-
-	/* bitmap of priorities that contain at least one ready thread */
-	u32_t prio_bmap[K_NUM_PRIO_BITMAPS];
 #endif
 
-	/* ready queues, one per priority */
-	sys_dlist_t q[K_NUM_PRIORITIES];
+#ifdef CONFIG_SCHED_DUMB
+	sys_dlist_t runq;
+#else
+	struct _priq_rb runq;
+#endif
 };
 
 typedef struct _ready_q _ready_q_t;
@@ -90,6 +93,9 @@ struct _cpu {
 	/* currently scheduled thread */
 	struct k_thread *current;
 
+	/* one assigned idle thread per CPU */
+	struct k_thread *idle_thread;
+
 	int id;
 };
 
@@ -158,8 +164,10 @@ typedef struct _kernel _kernel_t;
 extern struct _kernel _kernel;
 
 #ifdef CONFIG_SMP
+#define _current_cpu (_arch_curr_cpu())
 #define _current (_arch_curr_cpu()->current)
 #else
+#define _current_cpu (&_kernel.cpus[0])
 #define _current _kernel.current
 #endif