Drop the TARGETPLATFORM=BUILD indication of stuff that is to be built
for the build platform but pointless to build for the host platform. I
will handle the split of stuff built for the build or host platforms
differently. Note that some libraries need to be built for both
platforms.

Add explicit rules to do nothing for the cross-compilation case, but
likely even that will be unnecessary in the case of complete modules
like soltools (?). I will just mark modules that are for the build
platform only with an own flag in BUILD_TYPE.