Add script to prepare L4T image for OSS graphics stack compilation

prepare-rootfs will perform the following:

- Download and install qemu-static-arm into the target FS to allow to
  run ARM binaries from chroot on an x86 host
- Temporarily replace the target FS's resolv.conf to enable network
  access
- Chroot into the target FS and run apt-get to download the dependencies
  required for the OSS graphics stack compilation
- Fix the absolute libraries symlinks so they can be resolved during
  cross-compilation.
weston: set permissions of weston-launch after install
Make script execution stop in case of error
Indicate success at the end of each compilation script
Cleanup build scripts and add success message

Make the configuration options more readable, factorize some code, and
indicate successful installation of packages.