BUGFIX: Enabled BLE_COMMON_OPT_RADIO_CPU_MUTEX option
BUGFIX: Enabled BLE_COMMON_OPT_RADIO_CPU_MUTEX option

This option is now configured when the BLE stack is initialised. This ensures that SoftDevice is never
starved of CPU during periods of criticality. This does lock out the CPU for application use, but prevents
MIC failures caused by __disable_irq() operations (as found in the mbed Ticker API for example).

markshannon/microbit-dal,markshannon/microbit-dal