tests/bsim compile: Do not provide prj conf if default

Do not provide the defautl prj.conf to cmake, as that
causes it to not use the board overlays, which is not
what users would normally expect.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

tests/bsim/compile.source

@@ -39,8 +39,11 @@ function _compile(){
 
   local ret=0
 
-  local cmake_cmd=(cmake -GNinja -DBOARD_ROOT=${BOARD_ROOT} -DBOARD=${BOARD} \
-            -DCONF_FILE=${conf_file} -DOVERLAY_CONFIG=${conf_overlay} \
+  local cmake_cmd=(cmake -GNinja -DBOARD_ROOT=${BOARD_ROOT} -DBOARD=${BOARD})
+  if [ $conf_file != "prj.conf" ]; then
+    local cmake_cmd+=( -DCONF_FILE=${conf_file})
+  fi
+  local cmake_cmd+=( -DOVERLAY_CONFIG=${conf_overlay} \
             ${modules_arg} \
             ${cmake_args} -DCMAKE_C_FLAGS=\"${cc_flags}\" ${app_root}/${app})