.travis.yml: Move Contiki and LwIP to separate stage

Special handling of platforms other than POSIX reduces the build
matrix and thus speeds up CI once additional configure options
need to be checked.

.travis.yml

@@ -13,8 +13,6 @@ env:
   - PLATFORM=posix TESTS=yes TLS=openssl
   - PLATFORM=posix TESTS=yes TLS=gnutls
   - PLATFORM=posix TESTS=yes TLS=tinydtls
-  - PLATFORM=contiki TLS=no
-  - PLATFORM=lwip TLS=no
 
 matrix:
   exclude:
@@ -22,10 +20,6 @@ matrix:
       env: PLATFORM=posix TESTS=yes TLS=no
     - compiler: clang
       env: PLATFORM=posix TESTS=yes TLS=tinydtls
-    - compiler: clang
-      env: PLATFORM=contiki TLS=no
-    - compiler: clang
-      env: PLATFORM=lwip TLS=no
 
 before_install:
   - docker build -t obgm/libcoap:travis-env .
@@ -39,10 +33,23 @@ branches:
 
 stages:
   - test
+  - other platforms
   - dist
 
 jobs:
   include:
+    - stage: other platforms
+      compiler: gcc
+      env:
+        - PLATFORM=contiki TLS=no
+      script:
+        - docker run -e CC -e PLATFORM -e TLS obgm/libcoap:travis-env /bin/sh -c "scripts/build.sh"
+    - stage: other platforms
+      compiler: gcc
+      env:
+        - PLATFORM=lwip TLS=no
+      script:
+        - docker run -e CC -e PLATFORM -e TLS obgm/libcoap:travis-env /bin/sh -c "scripts/build.sh"
     - stage: dist
       compiler: gcc
       env: