kernel: pipes: add pipe flush routines

Adds two routines to flush pipe objects:
   k_pipe_flush()
     - This routine flushes the entire pipe. That includes both
     the pipe's buffer and all pended writers. It is equivalent
     to reading everything into a giant temporary buffer which
     is then discarded.
   k_pipe_buffer_flush()
     - This routine flushes only the pipe's buffer (if it exists).
     It is equivalent to reading a maximum of "buffer size" bytes
     into a temporary buffer which is then discarded.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>

include/kernel.h

@@ -4925,6 +4925,35 @@ __syscall size_t k_pipe_read_avail(struct k_pipe *pipe);
  */
 __syscall size_t k_pipe_write_avail(struct k_pipe *pipe);
 
+/**
+ * @brief Flush the pipe of write data
+ *
+ * This routine flushes the pipe. Flushing the pipe is equivalent to reading
+ * both all the data in the pipe's buffer and all the data waiting to go into
+ * that pipe into a large temporary buffer and discarding the buffer. Any
+ * writers that were previously pended become unpended.
+ *
+ * @param pipe Address of the pipe.
+ *
+ * @return N/A
+ */
+__syscall void k_pipe_flush(struct k_pipe *pipe);
+
+/**
+ * @brief Flush the pipe's internal buffer
+ *
+ * This routine flushes the pipe's internal buffer. This is equivalent to
+ * reading up to N bytes from the pipe (where N is the size of the pipe's
+ * buffer) into a temporary buffer and then discarding that buffer. If there
+ * were writers previously pending, then some may unpend as they try to fill
+ * up the pipe's emptied buffer.
+ *
+ * @param pipe Address of the pipe.
+ *
+ * @return N/A
+ */
+__syscall void k_pipe_buffer_flush(struct k_pipe *pipe);
+
 /** @} */
 
 /**