add .travis.yml to run make download check on every package

Travis will run the following steps for every new or modified package:

- mkdir tempdir
- unpack SDK
- make download
- make check

make download will download the source code.
make check will do certain checks. Atm only checking the validity of
PKG_HASH.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

.travis.yml

@@ -0,0 +1,12 @@
+language: c
+dist: trusty
+sudo: false
+cache:
+  directories:
+    - $HOME/sdk
+
+before_script:
+  - ./.travis_do.sh download_sdk
+
+script:
+  - ./.travis_do.sh test_packages