Fixed L6235E link error for NRF51822 variants

Changed TARGET_NRF51822 target directory names to TARGET_MCU_NRF51822
Removed NRF51822 names to extra_label of target.py
Added MCU_NRF51822 names to extra_label of target.py
Tested with blinky example for NRF51822, HRM1017, ARCH_BLE and
RBLAB_NRF51822 target