travis: Build more boards as part of nrf job.

Specifically:
- pca10040: It was already the default.
- microbit: It uses nRF51, has a Cortex-M0, and has additional libraries.
- pca10056: It has USB CDC.

.travis.yml

@@ -296,7 +296,9 @@ jobs:
         - arm-none-eabi-gcc --version
       script:
         - make ${MAKEOPTS} -C ports/nrf submodules
-        - make ${MAKEOPTS} -C ports/nrf
+        - make ${MAKEOPTS} -C ports/nrf BOARD=pca10040
+        - make ${MAKEOPTS} -C ports/nrf BOARD=microbit
+        - make ${MAKEOPTS} -C ports/nrf BOARD=pca10056
 
     # bare-arm and minimal ports, with size-diff check
     - stage: test