Workaround: Disable AN521_ARMCLANG_IPC_1_STORAGE_Debug_BL2_NS

AN521_ARMCLANG_IPC_1_STORAGE_Debug_BL2_NS does not work as expected
with ARMCLANG v6.13. Disable it for the time being to avoid blocking
Open CI. This patch will be reverted once the issue is figured out.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I2c711bdb2867c6ff633a8fc64f232d0b29907b5f
diff --git a/build_helper/build_helper_configs.py b/build_helper/build_helper_configs.py
index 1f55a1c..6a47eaf 100755
--- a/build_helper/build_helper_configs.py
+++ b/build_helper/build_helper_configs.py
@@ -142,6 +142,9 @@
 
 # List of all build configs that are impossible under all circumstances
 _common_tfm_invalid_configs = [
+    # AN521_ARMCLANG_IPC_1_STORAGE_Debug_BL2_NS does not work as expected with ARMCLANG v6.13
+    ("arm/mps2/an521", "toolchain_ARMCLANG.cmake", False, "1", False, "STORAGE",
+     "Debug", "off", True, True, "", "ON", ""),
     # LR_CODE size exceeds limit on MUSCA_B1 & MUSCA_S1 with regression tests in Debug mode built with ARMCLANG
     ("arm/musca_b1/sse_200", "toolchain_ARMCLANG.cmake", "*", "*", True, "OFF", "Debug", "*", "*", "*", "", "*", "*"),
     ("arm/musca_s1", "toolchain_ARMCLANG.cmake", "*", "*", True, "OFF", "Debug", "*", "*", "*", "", "*", "*"),