travis: Add zephyr build to CI.

Adds a job to build the zephyr port in CI using the same docker container
that the zephyr project uses for its own CI.

Always make clean zephyr builds to ensure we don't just rebuild C code, but
we also rebuild Kconfig and dts.  This is required when switching between
boards, which have different Kconfigs and device trees.

Uses the tagged zephyr 2.3.0 release.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>

.travis.yml

@@ -37,6 +37,33 @@ jobs:
         - tools/codeformat.py
         - git diff --exit-code
 
+    # zephyr port
+    - stage: test
+      name: "zephyr port build"
+      services:
+        - docker
+      before_install:
+        - docker pull zephyrprojectrtos/ci:v0.11.8
+        - >
+          docker run --name zephyr-ci -d -it
+          -v "$(pwd)":/micropython
+          -e ZEPHYR_SDK_INSTALL_DIR=/opt/sdk/zephyr-sdk-0.11.3
+          -e ZEPHYR_TOOLCHAIN_VARIANT=zephyr
+          -w /micropython/ports/zephyr
+          zephyrprojectrtos/ci:v0.11.8
+        - docker ps -a
+      install:
+        - docker exec zephyr-ci west init --mr v2.3.0 /zephyrproject
+        - docker exec -w /zephyrproject zephyr-ci west update
+        - docker exec -w /zephyrproject zephyr-ci west zephyr-export
+      script:
+        - docker exec zephyr-ci bash -c "make clean; ./make-minimal ${MAKEOPTS}"
+        - docker exec zephyr-ci bash -c "make clean; ./make-minimal ${MAKEOPTS} BOARD=frdm_k64f"
+        - docker exec zephyr-ci bash -c "make clean; make ${MAKEOPTS}"
+        - docker exec zephyr-ci bash -c "make clean; make ${MAKEOPTS} BOARD=frdm_k64f"
+        - docker exec zephyr-ci bash -c "make clean; make ${MAKEOPTS} BOARD=mimxrt1050_evk"
+        - docker exec zephyr-ci bash -c "make clean; make ${MAKEOPTS} BOARD=reel_board"
+
     # unix port on OSX (first in list because the build VM takes a long time to start)
     - stage: test
       os: osx