webkitdevci image: optimize list of packages and add quirk for RPi NFS boot.

[clopez]

Some changes to improve the webkitdevci image:

• Optimize the list of packages that go into the image: remove big unneeded stuff like mariadb/mysql or LTP tests

• Remove also splash: better to see the logs of the init process.

• Add some packages needed to build cog and useful utilities.

• Switch default compression to xz (0 level): is faster than bzip2 and compresses even better.

• Add .tar.xz as a default image format for this image (it will be the format used in the WebKit CI for the bots) and set WIC to use xz compression as well.

• Add a quirk for RPi that will copy into the /boot directory all the files needed to actually boot the RPi. This is very useful to create an image tarball (format .tar.xz for example) that is ready to boot via NFS instead of having to deal with individual boot files not packages inside the tarball.

[clopez]

This changes will be used with the script cross-toolchain-helper on an upcoming PR for WebKit. But first merge this if possible to avoid carrying an extra patch there.

[clopez]

merged in d82187df5dd9b7e2687ef7517544c6209f518ddf