Description: Fix undefined board_chaintool_type resulting in compilation error

IssueNo: https://gitee.com/openharmony/third_party_cJSON/issues/I5ODQ6
Feature or Bugfix: Bugfix
Binary Source: No
Signed-off-by: Nsuwenxiang <suwenxiang@huawei.com>

BUILD.gn

@@ -36,7 +36,7 @@ if (defined(ohos_lite)) {
       target_type = "shared_library"
       deps = [ "//build/lite/config/component/zlib:zlib_shared" ]
     }
-    if (board_toolchain_type == "iccarm") {
+    if (defined(board_toolchain_type) && board_toolchain_type == "iccarm") {
       cflags = [
         "--diag_suppress",
         "Pa082,Pa084",