kernel: select work queue implementation

Attempts to reimplement the existing work API using a new work
implementation failed, primarily due to heavy use of whitebox testing
in validating the original API.  Add a temporary Kconfig that will
select between the two implementations so we can use the same
identifiers but select which implementation they reference.

This commit just adds the selection infrastructure and uses it to
conditionalize the existing implementation in anticipation of the new
one in the next commit.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

include/kernel.h

@@ -2482,6 +2482,8 @@ __syscall int k_stack_pop(struct k_stack *stack, stack_data_t *data,
 
 /** @} */
 
+#ifdef CONFIG_KERNEL_WORK1
+
 struct k_work;
 
 /**
@@ -2954,6 +2956,9 @@ static inline int32_t k_delayed_work_remaining_get(const struct k_delayed_work *
 }
 
 /** @} */
+
+#endif /* CONFIG_KERNEL_WORK1 */
+
 /**
  * @defgroup mutex_apis Mutex APIs
  * @ingroup kernel_apis