.travis.yml: use latest device tree compiler

- use latest device tree compiler (suggested by Daniel Schwierzeck)
- add sandbox
- seperate echo and execution of TEST_CMD
- use curly braces
- remove BUILD_NCPUS( part of script)
- introduce ${TEST_CONFIG_CMD}
- add libsdl-dev package which is required by sandbox
- buildman initial try
- disable powerpc, it takes too long
- use --inline-suppr option for cppcheck

Signed-off-by: Roger Meier <r.meier@siemens.com>