I've built the ports/nrf for microbit with default configuration :

[nrf]$ make BOARD=microbit clean
[nrf]$ make -j8 BOARD=microbit

and using $ screen /dev/ttyACM0 115200 to access REPL, but have some issues :

• file system microbitfs is not accessible via ufs or Mu Editor. How to externally acess the internal file system ? With official MicroPython for BBC micro:bit ufs and Mu Editor can access the files;
• Ctrl+d in REPL freezes, as well as machine.soft_reset();
• deepsleep() and lightsleep() don't work. It seems that __WFIand __WFE are not working on microbit/nRF51822;
• unstable I2C scan reading.

And building with Bluetooth :

[nrf]$ make BOARD=microbit clean
[nrf]$ make -j8 BOARD=microbit SD=s110

then REPL is not accessible at all via $ screen /dev/ttyACM0 115200.

Not issues, but features lacking with respect to official MicroPython for BBC Micro:bit :

• no float numbers and math module, but it is possible to enable, see my commit "nrf : nRF51 with float enabled" in my branch;
• module 'microbit' :
  • no button_a.*/button_b.*;
  • no display.read_light_level();
  • no accelerometer.*;
  • no compass.*.

About the buttons A and B, it is simple to read via machine.Pin :

from machine import Pin
button_a = Pin(Pin.cpu.P17, Pin.IN, Pin.PULL_UP)
button_b = Pin(Pin.cpu.P26, Pin.IN, Pin.PULL_UP)
button_a.value()   # button A not pressed
    1
button_a.value()   # button A pressed
    0