mesa: tune configuration options

GLX can be enabled for the case we support X some day.
Use highlighted status macro to report status in scripts

This is more visible than a simple echo. Also only download the L4T root
FS and drivers pack if needed.
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.
Use same compilation scheme for Linux and Nouveau

Use the same compilation scheme as other packages for Linux and Nouveau,
namely:

- build into out/build/
- perform installation within build script
- install into $SYSROOT
- use the downloaded cross-compilation toolchain

Result boots and is usable, yay!