cmake: add helpers for setting board runners

This helps by letting us add checks for when the runner has already
been set. There is documentation saying you can set
-DBOARD_DEBUG_RUNNER at the command line and have it take effect,
which turns out not to be true for a large number of boards.
A status message helps the user debug.

(We'll address the existing in-tree boards in the next patch.)

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>

boards/arc/arduino_101_sss/board.cmake

@@ -1,12 +1,12 @@
 # SPDX-License-Identifier: Apache-2.0
 
 if(DEFINED ENV{ZEPHYR_FLASH_OVER_DFU})
-  set(BOARD_FLASH_RUNNER dfu-util)
+  board_set_flasher(dfu-util)
 else()
-  set(BOARD_FLASH_RUNNER openocd)
+  board_set_flasher(openocd)
 endif()
 
-set(BOARD_DEBUG_RUNNER openocd)
+board_set_debugger(openocd)
 
 board_runner_args(dfu-util "--pid=8087:0aba" "--alt=sensor_core")
 board_runner_args(openocd --cmd-pre-load "targets 1" "--gdb-port=3334")

boards/arc/em_starterkit/board.cmake

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 
 # TODO: can this board just use the usual openocd runner?
-set(BOARD_FLASH_RUNNER em-starterkit)
-set(BOARD_DEBUG_RUNNER em-starterkit)
+board_set_flasher(em-starterkit)
+board_set_debugger(em-starterkit)
 board_finalize_runner_args(em-starterkit)

boards/arc/iotdk/board.cmake

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 
 # TODO: can this board just use the usual openocd runner?
-set(BOARD_FLASH_RUNNER em-starterkit)
-set(BOARD_DEBUG_RUNNER em-starterkit)
+board_set_flasher(em-starterkit)
+board_set_debugger(em-starterkit)
 board_finalize_runner_args(em-starterkit)

boards/arc/nsim/board.cmake

@@ -2,8 +2,8 @@
 
 set(EMU_PLATFORM nsim)
 
-set(BOARD_FLASH_RUNNER arc-nsim)
-set(BOARD_DEBUG_RUNNER arc-nsim)
+board_set_flasher(arc-nsim)
+board_set_debugger(arc-nsim)
 
 if(${CONFIG_SOC_NSIM_EM})
 board_runner_args(arc-nsim "--props=nsim_em.props")