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>