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/userland/hello_cpp.cpp

@@ -0,0 +1,7 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#sanity-checks */
+
+#include <iostream>
+
+int main() {
+    std::cout << "hello cpp" << std::endl;
+}