Nordic "nRF52840 Dongle" Support

@urish I've had a go at compiling this but I've hit a wall and was wondering if you remember whether there was something extra that you did to get it to compile? I grabbed that branch from the repo, copied all of SDK_15.0.0 into targetlibs/nrf5x_15, copied all of the micro-ecc repository into the external/micro-ecc folder and then ran make clean && BOARD=NRF52840DK RELEASE=1 make. I'm running into this:

/tmp/ccwJqUDl.ltrans0.ltrans.o: In function `nrf_drv_uart_tx':
/home/steven/projects/Espruino-NRF52840/­targetlibs/nrf5x_15/integration/nrfx/leg­acy/nrf_drv_uart.h:516: undefined reference to `nrfx_uarte_tx'
/tmp/ccwJqUDl.ltrans0.ltrans.o: In function `nrf_drv_uart_tx_abort':
/home/steven/projects/Espruino-NRF52840/­targetlibs/nrf5x_15/integration/nrfx/leg­acy/nrf_drv_uart.h:549: undefined reference to `nrfx_uarte_tx_abort'
/tmp/ccwJqUDl.ltrans9.ltrans.o: In function `nrf_drv_uart_uninit.constprop.241':
/home/steven/projects/Espruino-NRF52840/­targetlibs/nrf5x_15/integration/nrfx/leg­acy/nrf_drv_uart.h:466: undefined reference to `nrfx_uarte_uninit'
/tmp/ccwJqUDl.ltrans9.ltrans.o: In function `nrf_drv_uart_rx.constprop.202':
/home/steven/projects/Espruino-NRF52840/­targetlibs/nrf5x_15/integration/nrfx/leg­acy/nrf_drv_uart.h:565: undefined reference to `nrfx_uarte_rx'
/tmp/ccwJqUDl.ltrans9.ltrans.o: In function `nrf_drv_uart_init':
/home/steven/projects/Espruino-NRF52840/­targetlibs/nrf5x_15/integration/nrfx/leg­acy/nrf_drv_uart.c:124: undefined reference to `nrfx_uarte_init'
collect2: error: ld returned 1 exit status
make/targets/ARM.make:2: recipe for target 'espruino_1v99_nrf52840.elf' failed
make: *** [espruino_1v99_nrf52840.elf] Error 1

Which isn't making sense as sdk_config.h has this:

[#define](https://forum.espruino.com/sea­rch/?q=%23define) NRFX_UARTE_ENABLED 0
[#define](https://forum.espruino.com/sea­rch/?q=%23define) NRFX_UARTE0_ENABLED 0