nrf52dk: add BLE/6lo support utilizing Nordic's SoftDevice

Makefile.dep

boards/nrf52dk/Makefile.dep

boards/nrf52dk/Makefile.include

boards/nrf52dk/include/periph_conf.h

cpu/nrf52/cpu.c

cpu/nrf52/include/cpu_conf.h

cpu/nrf52/ldscripts/nrf52xxaa_sd.ld

cpu/nrf52/vectors.c

pkg/nordic_softdevice_ble/Makefile

pkg/nordic_softdevice_ble/Makefile.include

pkg/nordic_softdevice_ble/Makefile.module

pkg/nordic_softdevice_ble/README-BLE-6LoWPAN.md

pkg/nordic_softdevice_ble/README.md

pkg/nordic_softdevice_ble/src/Makefile

pkg/nordic_softdevice_ble/src/app_error.c

pkg/nordic_softdevice_ble/src/ble-core.c

pkg/nordic_softdevice_ble/src/ble-core.h

pkg/nordic_softdevice_ble/src/ble-mac.c

pkg/nordic_softdevice_ble/src/ble-mac.h

pkg/nordic_softdevice_ble/src/gnrc_nordic_ble_6lowpan.c