trace: add support for 'trace wipe'

trace: add support for 'trace wipe'

Implement a 'trace wipe' command to delete the currently accumulated
trace data. This comes handy when someone needs to trace a particular
command. For example:

  => trace pause; trace wipe
  => trace resume; dhcp; trace pause
  => trace stats
  => trace calls 0x02100000 0x10000000
  => tftpput $profbase $profoffset 192.168.0.16:trace.bin

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>