TFM: Remove BL2 macro workaround for mbed-cli1/ARMClang

We worked around an issue with mbed-cli1 not recognising the BL2 macro
from targets.json by adding patched versions of region_defs.h and
flash_layout.h for ARM_MUSCA targets. In the patched headers we defined
the BL2 macro to ensure it can be picked up by the ARM scatter files
that include the headers.

The current solution is not robust, because it means that the
aforementioned headers easily become out of date. A workaround of
defining the macros in the scatter files which need them was suggested
in https://github.com/ARMmbed/mbed-os/issues/14762

This commit applies the suggested changes to the ARM_MUSCA scatter
files.

targets/TARGET_ARM_SSG/TARGET_MUSCA_B1/device/TOOLCHAIN_ARMC6/musca_ns.sct

@@ -18,6 +18,10 @@
  * limitations under the License.
  */
 
+#if !defined(BL2)
+  #define BL2
+#endif
+
 #include "../../partition/region_defs.h"
 #include "../cmsis_nvic.h"
 

targets/TARGET_ARM_SSG/TARGET_MUSCA_S1/device/TOOLCHAIN_ARMC6/musca_ns.sct

@@ -18,6 +18,10 @@
  * limitations under the License.
  */
 
+#if !defined(BL2)
+  #define BL2
+#endif
+
 #include "../../partition/region_defs.h"
 #include "../cmsis_nvic.h"