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.