After using agent, we need to run testsuite in two cases, agent is on
and off.  This patch is to add such logic in testsuite, so that we can
control this through board file.  If we add the following line in board
file,

  set_board_info use_agent "on"

the agent is always turned on for remote testing.  We need also to turn
agent on in native debugging, but I'll postpone it when gdb starts to
use agent.

-- 
Yao (齐尧)