boards: enable mdb runner for arc boards

* current supported boards:
   * emsk, iotdk, nsim, emsdp, hsdk.
* for the unsupported future boards, pls take a
  reference of supported boards' board.cmake.
* mdb runner is required and the default runner for SMP
  case, e.g., HSDK and nsim_hs_smp.
* other ARC boards can also choose to use mdb by
  setting runner as mdb, e.g. west flash --runner mdb.
* with mdb runner, user can make a debug through mdb gui
* with arc_nsim or opencod runner (default runner), user
  can make a debug through gdb cmdline.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>