The unified NRF51 target and feature BLE directories have been reorganized to follow the naming and directory structure of the NRF52 implementation. This reorganization does not include TARGET_MCU_NRF51822 and derived targets.
Applying PR to NRF52 SDK 14.2: https://github.com/ARMmbed/mbed-os/pull/5046
Copied from previous location in targets/TARGET_NORDIC/TARGET_NRF5 Core functionality is working but some peripherals do not.