modules: rename packages/kernel_modules to packages/lkmc

Then inside, split packages/lkmc into kernel_modules and userland,
to keep userland out of the kernel_modules parent path, which makes no
sense.

Copy built modules and userland to the output rootfs overlay.

Document Linux distro tradeoffs.

packages/lkmc/anonymous_inode.h

@@ -0,0 +1,9 @@
+#ifndef IOCTL_H
+#define IOCTL_H
+
+#include <linux/ioctl.h>
+
+#define LKMC_ANONYMOUS_INODE_MAGIC 0x33
+#define LKMC_ANONYMOUS_INODE_GET_FD _IOR(LKMC_ANONYMOUS_INODE_MAGIC, 0, int)
+
+#endif