travis: Scan patch codestyle

Checkpatch is a tool which checks patch code style
alignment with Linux kernel code.
By the way print short graphs of used commits, to
show whats under test.
Checkpatch call is precede by `set -x`, to show used
arguments values, especially checked commits range.

Signed-off-by: Karol Trzcinski <karolx.trzcinski@linux.intel.com>

.travis.yml

@@ -13,3 +13,10 @@ jobs:
       script:
         - echo -e '#!/bin/bash\nmkdir build && cd build && cmake .. && make -Werror -Wall -Wmissing-prototypes -Wimplicit-fallthrough=3 -Wpointer-arith' > build.sh && chmod +x build.sh
         - docker run -i -t -v $(pwd):/home/sof/work/sof.git --user $(id -u) sof ./build.sh
+    - name: checkpatch
+      before_install:
+        - sudo apt-get -y install codespell
+      script:
+        - git --no-pager log --oneline --graph --decorate --max-count=5
+        - git --no-pager log --oneline --graph --decorate --max-count=5 "${TRAVIS_BRANCH}"
+        - (set -x; scripts/checkpatch.pl --no-tree --strict --codespell --no-signoff -g ${TRAVIS_COMMIT_RANGE/.../..})