Remove note on cross-built kernels

[koppi]

In machinekit-ci/rpi_linux@afaec88 I have fixed the issue with cross built kernels and x64-64 vs. ARM executables in cross-built linux-header Debian packages.

@mhaberler please remove the note on cross-built kernels in the README.md and adapt all the other kernel package Jenkins cross build scripts to make the dkms mechanism work properly with cross-built linux-header packages.

[mhaberler]

the socfpga kernel is fine wrt out-of-tree and dkms module builds.

thanks! yes, that was so-so text.