ch5-kernel make mrproper explanation

[gerard@…]

it says "to ensure the tree is clean". It's the only package where we run a 'make clean' type of command, so we should add why it's run here specifically. One point is that the kernel docs suggest it. Any other reasons. I suppose we could assume it's clean, since we assume this for every other package, and just add a mrproper when we detect a kernel version that's not 'clean'.

Run this by the lfs-dev before actually making this change

[markh@…]

The kernel is *so* important and as they recommend running make mrproper I don't see any harm in continuing to do it. Suggest we just keep it and add a note to this effect.