Build: Rename TFM_PLATFORM

Arm platforms are moved into arm folder, so the build command
TFM_PLATFORM should do the corresponding changes.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: Iba23e4f71d5c5865f358c2b7ce612ba603eff4e7
diff --git a/memory_footprint.py b/memory_footprint.py
index 1a24fa2..4145f22 100644
--- a/memory_footprint.py
+++ b/memory_footprint.py
@@ -175,7 +175,7 @@
         REFERENCE_CONFIGS[i] = REFERENCE_CONFIGS[i].strip().lower()
     config = identify_config()
     if (config[2].lower() in REFERENCE_CONFIGS
-        and config[0] == "mps2/an521"
+        and config[0] == "arm/mps2/an521"
         and config[1] == "toolchain_GNUARM.cmake"):
         # Pushing data for AN521 and GNUARM
         print("Configuration " + config[2] + " is a reference")