target: Add BL2 macro to MUSCA B target config

BL2 macro is used in `region_defs.h` to define the `BL2_HEADER_SIZE`.
Without BL2 macro, `BL2_HEADER_SIZE` is set to 0. This leads to
incorrect start address (Reset_Handler of Mbed OS) derived by TF-M based
on `region_defs.h` and BL2 macro.

BL2 macro is set for MUSCA B in TF-M.

Signed-off-by: Devaraj Ranganna <[email protected]>

Author: urutva

targets/targets.json

@@ -4799,7 +4799,8 @@
             "MBED_FAULT_HANDLER_DISABLED",
             "CMSIS_NVIC_VIRTUAL",
             "LPTICKER_DELAY_TICKS=3",
-            "MBED_MPU_CUSTOM"
+            "MBED_MPU_CUSTOM",
+            "BL2"
         ],
         "extra_labels_add": [
             "MUSCA_B1_NS",