.travis.yml: build u-boot on travis-ci

Goal:
  - building all variants of U-Boot with multiple configurations
  - code quality checks and metrics
  - https://travis-ci.org/u-boot/u-boot/builds

---
Changes for v4:
  - ignore HACKKIT board (suggested by Wolfgang Denk)
  - add /tmp/dtc as first element to PATH (suggested by Wolfgang Denk)

Changes for v3:
  - fix patch format (recognized by Andreas Färber)
  - get in touch with GitHub about the orphan u-boot account and get it
  - install travis-ci hook on the account https://github.com/u-boot/u-boot/
  - push master and travis branch to https://github.com/u-boot/u-boot/
  - add Goal to the commit message

Changes for v2:
  - use latest device tree compiler (suggested by Daniel Schwierzeck)
  - add sandbox (suggested by Daniel Schwierzeck)
  - seperate echo and execution of TEST_CMD
  - remove BUILD_NCPUS( part of script)
  - introduce ${TEST_CONFIG_CMD}
  - add libsdl-dev package which is required by sandbox
  - use buildman (suggested by Tom Rini)
  - generate ~/.buildman
  - split ppc into several jobs
  - install gcc-arm-linux-gnueabihf
  - use --inline-suppr option for cppcheck

Signed-off-by: Roger Meier <r.meier@siemens.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Andreas Färber <afaerber@suse.de>