target: Add BL2 macro to MUSCA B target config
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 <53e2fcb007e5c6e2caf46b91114119a655266f64@arm.com>

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed