travis: For powerpc job, build both UART variants.

The powerpc port can be built with two different UART drivers, so build
both in CI.

The default compiler is now powerpc64le-linux-gnu- so it does not need to
be specified on the command line.

.travis.yml

@@ -355,4 +355,5 @@ jobs:
       install:
         - sudo apt-get install gcc-powerpc64le-linux-gnu libc6-dev-ppc64el-cross
       script:
-        - make ${MAKEOPTS} -C ports/powerpc CROSS_COMPILE=powerpc64le-linux-gnu-
+        - make ${MAKEOPTS} -C ports/powerpc UART=potato
+        - make ${MAKEOPTS} -C ports/powerpc UART=lpc_serial