cmake: flash: support verbose output toggle

If CMAKE_VERBOSE_MAKEFILE is defined, make flashing and debugging more
verbose.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
diff --git a/cmake/flash/CMakeLists.txt b/cmake/flash/CMakeLists.txt
index 153a1ea..971d955 100644
--- a/cmake/flash/CMakeLists.txt
+++ b/cmake/flash/CMakeLists.txt
@@ -14,6 +14,13 @@
   "--openocd-search=${OPENOCD_DEFAULT_PATH}"
   )
 
+# Enable verbose output, if requested.
+if(CMAKE_VERBOSE_MAKEFILE)
+  set(RUNNER_VERBOSE "--verbose")
+else()
+  set(RUNNER_VERBOSE)
+endif()
+
 foreach(target flash debug debugserver)
   string(TOUPPER "${target}" target_upper)
 
@@ -42,6 +49,7 @@
       ${CMAKE_COMMAND} -E env
       ${PYTHON_EXECUTABLE}
       $ENV{ZEPHYR_BASE}/scripts/support/zephyr_flash_debug.py
+      ${RUNNER_VERBOSE}
       ${runner}
       ${target}
       ${RUNNER_ARGS_COMMON}