ARM_MUSCA_B1: default baud rate to 115200
ARM_MUSCA_B1: default baud rate to 115200

The TF-M secure binary has a fixed baud rate of 115200. Having a
different baud rate on the non-secure side results in broken serial
outputs.

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