Clean kernel source directory before setting up make alias

When kbuild is using a separate directory for output files then it will check if the kernel source directory is clean. The build will throw an error when the source directory is not clean:

.. is not clean, please run 'make mrproper'

Once envkernel.sh has aliased the make command then make mrproper will be run in the context of .output directory. If the source directory is not clean then user may be wondering why make mrproper doesn't clean it.

This change will ensure that the source directory is clean when envkernel is setup.