Yocto build: unify image and SDK build jobs

Image and SDK builds share a lot of similar code. Instead of having two job classes build_yocto_image and build_yocto_sdk for these two tasks, merge them into the base class build_yocto and make the differences configurable via a variable.

The dump_install_command part of the script, which was not executed for SDK builds, is always present now, but it's only executed if the INSTALLSCRIPT variable is set, which is not the case for SDK builds.

The collect_srcrevs part of the script is executed in all cases. It was not part of the SDK build before, but it's not less relevant there.