image: always rebuild kernel loaders

image: always rebuild kernel loaders

Kernel loaders like the lzma-loader currently don't track changes to
their sources. This can lead to an old version of a loader to be used
when a build tree is not clean between builds.

As the loaders are tiny and the build times are insignificant, simply
force rebuilding them on every build to avoid this problem.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
(cherry picked from commit a01d23e755ba46f41e667d558d82d4871d7f5450)